(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/gen-server/entity/Document.ts

@@ -6,6 +6,7 @@ import {Column, Entity, JoinColumn, ManyToOne, OneToMany, PrimaryColumn} from "t
 import {AclRuleDoc} from "./AclRule";
 import {Alias} from "./Alias";
 import {Resource} from "./Resource";
+import {Secret} from "./Secret";
 import {Workspace} from "./Workspace";
 
 // Acceptable ids for use in document urls.
@@ -58,6 +59,9 @@ export class Document extends Resource {
   @Column({name: 'options', type: nativeValues.jsonEntityType, nullable: true})
   public options: DocumentOptions | null;
 
+  @OneToMany(_type => Secret, secret => secret.doc)
+  public secrets: Secret[];
+
   public checkProperties(props: any): props is Partial<DocumentProperties> {
     return super.checkProperties(props, documentPropertyKeys);
   }

app/gen-server/entity/Secret.ts

@@ -0,0 +1,16 @@
+import {BaseEntity, Column, Entity, JoinColumn, ManyToOne, PrimaryColumn} from "typeorm";
+import {Document} from "./Document";
+
+@Entity({name: 'secrets'})
+export class Secret extends BaseEntity {
+  @PrimaryColumn()
+  public id: string;  // generally a UUID
+
+  @Column({name: 'value'})
+  public value: string;
+
+  @ManyToOne(_type => Document, { onDelete: 'CASCADE' })
+  @JoinColumn({name: 'doc_id'})
+  public doc: Document;
+
+}