(core) Initial webhooks implementation

Summary:
See https://grist.quip.com/VKd3ASF99ezD/Outgoing-Webhooks

- 2 new DocApi endpoints: _subscribe and _unsubscribe, not meant to be user friendly or publicly documented. _unsubscribe should be given the response from _subscribe in the body, e.g:

```
$ curl -X POST -H "Authorization: Bearer 8fd4dc59ecb05ab29ae5a183c03101319b8e6ca9" "http://localhost:8080/api/docs/6WYa23FqWxGNe3AR6DLjCJ/tables/Table2/_subscribe" -H "Content-type: application/json" -d '{"url": "https://webhook.site/a916b526-8afc-46e6-aa8f-a625d0d83ec3", "eventTypes": ["add"], "isReadyColumn": "C"}'
{"unsubscribeKey":"3246f158-55b5-4fc7-baa5-093b75ffa86c","triggerId":2,"webhookId":"853b4bfa-9d39-4639-aa33-7d45354903c0"}
$ curl -X POST -H "Authorization: Bearer 8fd4dc59ecb05ab29ae5a183c03101319b8e6ca9" "http://localhost:8080/api/docs/6WYa23FqWxGNe3AR6DLjCJ/tables/Table2/_unsubscribe" -H "Content-type: application/json" -d '{"unsubscribeKey":"3246f158-55b5-4fc7-baa5-093b75ffa86c","triggerId":2,"webhookId":"853b4bfa-9d39-4639-aa33-7d45354903c0"}'
{"success":true}
```

- New DB entity Secret to hold the webhook URL and unsubscribe key
- New document metatable _grist_Triggers subscribes to table changes and points to a secret to use for a webhook
- New file Triggers.ts processes action summaries and uses the two new tables to send webhooks.
- Also went on a bit of a diversion and made a typesafe subclass of TableData for metatables.

I think this is essentially good enough for a first diff, to keep the diffs manageable and to talk about the overall structure. Future diffs can add tests and more robustness using redis etc. After this diff I can also start building the Zapier integration privately.

Test Plan: Tested manually: see curl commands in summary for an example. Payloads can be seen in https://webhook.site/#!/a916b526-8afc-46e6-aa8f-a625d0d83ec3/0b9fe335-33f7-49fe-b90b-2db5ba53382d/1 . Great site for testing webhooks btw.

Reviewers: dsagal, paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D3019

app/common/DocData.ts

@@ -3,13 +3,13 @@
  * subscribes to actions which change it, and forwards those actions to individual tables.
  * It also provides the interface to apply actions to data.
  */
-import {schema} from 'app/common/schema';
+import {schema, SchemaTypes} from 'app/common/schema';
 import fromPairs = require('lodash/fromPairs');
 import groupBy = require('lodash/groupBy');
 import {ActionDispatcher} from './ActionDispatcher';
 import {BulkColValues, ColInfo, ColInfoWithId, ColValues, DocAction,
         RowRecord, TableDataAction} from './DocActions';
-import {ColTypeMap, TableData} from './TableData';
+import {ColTypeMap, MetaTableData, TableData} from './TableData';
 
 type FetchTableFunc = (tableId: string) => Promise<TableDataAction>;
 
@@ -46,7 +46,7 @@ export class DocData extends ActionDispatcher {
    * Creates a new TableData object. A derived class may override to return an object derived from TableData.
    */
   public createTableData(tableId: string, tableData: TableDataAction|null, colTypes: ColTypeMap): TableData {
-    return new TableData(tableId, tableData, colTypes);
+    return new (tableId in schema ? MetaTableData : TableData)(tableId, tableData, colTypes);
   }
 
   /**
@@ -56,6 +56,13 @@ export class DocData extends ActionDispatcher {
     return this._tables.get(tableId);
   }
 
+  /**
+   * Like getTable, but the result knows about the types of its records
+   */
+  public getMetaTable<TableId extends keyof SchemaTypes>(tableId: TableId): MetaTableData<TableId> {
+    return this.getTable(tableId) as any;
+  }
+
   /**
    * Returns an unsorted list of all tableIds in this doc, including both metadata and user tables.
    */

app/common/TableData.ts

@@ -7,6 +7,7 @@ import {ActionDispatcher} from './ActionDispatcher';
 import {BulkColValues, CellValue, ColInfo, ColInfoWithId, ColValues, DocAction,
         isSchemaAction, ReplaceTableData, RowRecord, TableDataAction} from './DocActions';
 import {arrayRemove, arraySplice} from './gutil';
+import {SchemaTypes} from "./schema";
 
 export interface ColTypeMap { [colId: string]: string; }
 
@@ -470,6 +471,34 @@ export class TableData extends ActionDispatcher implements SkippableRows {
   }
 }
 
+export type MetaRowRecord<TableId extends keyof SchemaTypes> = SchemaTypes[TableId] & RowRecord;
+
+/**
+ * Behaves the same as TableData, but uses SchemaTypes for type safety of its columns.
+ */
+export class MetaTableData<TableId extends keyof SchemaTypes> extends TableData {
+  constructor(tableId: TableId, tableData: TableDataAction | null, colTypes: ColTypeMap) {
+    super(tableId, tableData, colTypes);
+  }
+
+  public getRecords(): Array<MetaRowRecord<TableId>> {
+    return super.getRecords() as any;
+  }
+
+  public getRecord(rowId: number): MetaRowRecord<TableId> | undefined {
+    return super.getRecord(rowId) as any;
+  }
+
+  /**
+   * Same as getRowPropFunc, but I couldn't get a direct override to compile.
+   */
+  public getMetaRowPropFunc<ColId extends keyof SchemaTypes[TableId]>(
+    colId: ColId
+  ): ((rowId: number | "new") => SchemaTypes[TableId][ColId]) {
+    return super.getRowPropFunc(colId as any) as any;
+  }
+}
+
 function reassignArray<T>(targetArray: T[], sourceArray: T[]): void {
   targetArray.length = 0;
   arraySplice(targetArray, 0, sourceArray);

app/common/schema.ts

@@ -143,6 +143,13 @@ export const schema = {
     timeUploaded        : "DateTime",
   },
 
+  "_grist_Triggers": {
+    tableRef            : "Ref:_grist_Tables",
+    eventTypes          : "ChoiceList",
+    isReadyColRef       : "Ref:_grist_Tables_column",
+    actions             : "Text",
+  },
+
   "_grist_ACLRules": {
     resource            : "Ref:_grist_ACLResources",
     permissions         : "Int",
@@ -317,6 +324,13 @@ export interface SchemaTypes {
     timeUploaded: number;
   };
 
+  "_grist_Triggers": {
+    tableRef: number;
+    eventTypes: ['L', ...string[]]|null;
+    isReadyColRef: number;
+    actions: string;
+  };
+
   "_grist_ACLRules": {
     resource: number;
     permissions: number;