(core) Adding /webhooks endpoint

Summary:
- New /webhooks event that lists all webhooks in a document (available for owners),
- Monitoring webhooks usage and saving it in memory or Redis,
- Loosening _usubscribe API endpoint, so that the information returned from the /webhook endpoint is enough to unsubscribe,
- Owners can remove webhook without the unsubscribe key.

The endpoint lists all webhooks that are registered in a document, not just webhooks from a single table.
There are two status fields. First for the webhook, second for the last request attempt.
Webhook can have 5 statuses: 'idle', 'sending', 'retrying', 'postponed', 'error', which roughly describes what the
sendLoop is currently doing. The 'error' status describes a situation when all request attempts failed and the queue needs
to be drained, so some requests were dropped.

The last request status can only be: 'success', 'failure' or 'rejected'. Rejected means that the last batch was dropped because the
queue was too long.

Test Plan: New and updated tests

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D3727

app/server/lib/ActiveDoc.ts

@@ -1727,6 +1727,13 @@ export class ActiveDoc extends EventEmitter {
     await this._triggers.clearWebhookQueue();
   }
 
+  /**
+   * Returns the list of outgoing webhook for a table in this document.
+   */
+  public async webhooksSummary() {
+    return this._triggers.summary();
+  }
+
   /**
    * Loads an open document from DocStorage.  Returns a list of the tables it contains.
    */