(core) add user and docId to heartbeat logging

Summary: add user and docId to heartbeat logging

Test Plan: checked manually

Reviewers: dsagal

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D2784

app/client/components/GristWSConnection.ts

@@ -274,6 +274,7 @@ export class GristWSConnection extends Disposable {
     this.send(JSON.stringify({
       beat: 'alive',
       url: G.window.location.href,
+      docId: this._assignmentId,
     }));
   }