(core) make document assignment endpoint available via /housekeeping api

Summary:
The /assign endpoint checks if a document is on the desired worker
and moves it if not. This is never done under regular operation, but
is useful when quarantining a misbehaving document.

The endpoint was failing to operate correctly if the requester did
not have access to the document. This diff makes the endpoint
accessible through a /housekeeping route, using the same pattern as
the /force-reload endpoint.

Test Plan: added test

Reviewers: dsagal

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D3109

app/gen-server/lib/Housekeeper.ts

@@ -161,6 +161,16 @@ export class Housekeeper {
         headers,
       });
     }));
+
+    // Move a document to its assigned worker.  Can be useful during administrative
+    // actions.
+    app.post('/api/housekeeping/docs/:docId/assign', this._withSupport(async (docId, headers) => {
+      const url = await this._server.getHomeUrlByDocId(docId, `/api/docs/${docId}/assign`);
+      return fetch(url, {
+        method: 'POST',
+        headers,
+      });
+    }));
   }
 
   /**