(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/gen-server/lib/DocApiForwarder.ts

@@ -55,6 +55,7 @@ export class DocApiForwarder {
     app.use('/api/docs/:docId/states', withDoc);
     app.use('/api/docs/:docId/compare', withDoc);
     app.use('/api/docs/:docId/assign', withDocWithoutAuth);
+    app.use('/api/docs/:docId/webhooks/queue', withDoc);
     app.use('^/api/docs$', withoutDoc);
   }