(core) get user.Name through same mechanism as user.id for websocket Client

Summary:
This avoids an extra database query to look up the user's current
name, by capturing it at the moment their user id is queried.

Test Plan: existing test for user.Name changes continues to pass

Reviewers: dsagal

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D3381

app/server/lib/FlexServer.ts

@@ -825,7 +825,6 @@ export class FlexServer implements GristServer {
       sessions: this._sessions,
       hosts: this._hosts,
       httpsServer: this.httpsServer,
-      dbManager: this._dbManager,
     });
   }
   /**