(core) Adding DELETE /api/docs/webhooks/queue endpoint to clear the queue

Summary:
Creating an API endpoint to cancel any queued webhook messages from
a document.

Test Plan: Updated

Reviewers: paulfitz, georgegevoian

Reviewed By: paulfitz, georgegevoian

Differential Revision: https://phab.getgrist.com/D3713

app/server/lib/ActiveDoc.ts

@@ -1712,6 +1712,13 @@ export class ActiveDoc extends EventEmitter {
     }
   }
 
+  /**
+   * Clears all outgoing webhook requests queued for this document.
+   */
+  public async clearWebhookQueue() {
+    await this._triggers.clearWebhookQueue();
+  }
+
   /**
    * Loads an open document from DocStorage.  Returns a list of the tables it contains.
    */