(core) Adds a UI panel for managing webhooks

Summary:
This adds a UI panel for managing webhooks. Work started by Cyprien Pindat. You can find the UI on a document's settings page. Main changes relative to Cyprien's demo:

  * Changed behavior of virtual table to be more consistent with the rest of Grist, by factoring out part of the implementation of on-demand tables.
  * Cell values that would create an error can now be denied and reverted (as for the rest of Grist).
  * Changes made by other users are integrated in a sane way.
  * Basic undo/redo support is added using the regular undo/redo stack.
  * The table list in the drop-down is now updated if schema changes.
  * Added a notification from back-end when webhook status is updated so constant polling isn't needed to support multi-user operation.
  *  Factored out webhook specific logic from general virtual table support.
  * Made a bunch of fixes to various broken behavior.
  * Added tests.

The code remains somewhat unpolished, and behavior in the presence of errors is imperfect in general but may be adequate for this case.

I assume that we'll soon be lifting the restriction on the set of domains that are supported for webhooks - otherwise we'd want to provide some friendly way to discover that list of supported domains rather than just throwing an error.

I don't actually know a lot about how the front-end works - it looks like tables/columns/fields/sections can be safely added if they have string ids that won't collide with bone fide numeric ids from the back end. Sneaky.

Contains a migration, so needs an extra reviewer for that.

Test Plan: added tests

Reviewers: jarek, dsagal

Reviewed By: jarek, dsagal

Differential Revision: https://phab.getgrist.com/D3856

app/common/Triggers.ts

@@ -4,8 +4,15 @@ export interface WebhookFields {
   tableId: string;
   enabled?: boolean;
   isReadyColumn?: string|null;
+  name?: string;
+  memo?: string;
 }
 
+// Union discriminated by type
+export type WebhookBatchStatus = 'success'|'failure'|'rejected';
+export type WebhookStatus = 'idle'|'sending'|'retrying'|'postponed'|'error'|'invalid';
+
+
 // WebhookSubscribe should be `Omit<WebhookFields, 'tableId'>` (because subscribe endpoint read
 // tableId from the url) but generics are not yet supported by ts-interface-builder
 export interface WebhookSubscribe {
@@ -13,6 +20,23 @@ export interface WebhookSubscribe {
   eventTypes: Array<"add"|"update">;
   enabled?: boolean;
   isReadyColumn?: string|null;
+  name?: string;
+  memo?: string;
+}
+
+export interface WebhookSummary {
+  id: string;
+  fields: {
+    url: string;
+    unsubscribeKey: string;
+    eventTypes: string[];
+    isReadyColumn: string|null;
+    tableId: string;
+    enabled: boolean;
+    name: string;
+    memo: string;
+  },
+  usage: WebhookUsage|null,
 }
 
 // Describes fields to update a webhook
@@ -29,4 +53,29 @@ export interface WebhookPatch {
   tableId?: string;
   enabled?: boolean;
   isReadyColumn?: string|null;
+  name?: string;
+  memo?: string;
+}
+
+
+export interface WebhookUsage {
+  // As minimum we need number of waiting events and status (by default pending).
+  numWaiting: number,
+  status: WebhookStatus;
+  updatedTime?: number|null;
+  lastSuccessTime?: number|null;
+  lastFailureTime?: number|null;
+  lastErrorMessage?: string|null;
+  lastHttpStatus?: number|null;
+  lastEventBatch?: null | {
+    size: number;
+    errorMessage: string|null;
+    httpStatus: number|null;
+    status: WebhookBatchStatus;
+    attempts: number;
+  },
+  numSuccess?: {
+    pastHour: number;
+    past24Hours: number;
+  },
 }