This PR updates Collab to make it validate access tokens through Cloud
instead of doing it in-house.
We're reusing the `GET /client/users/me` endpoint—which is what we also
call on the client—to validate the user's access token.
We only need to do this when establishing a WebSocket connection, so the
increased latency of a network hop shouldn't be a problem.
Closes CLO-308.
Release Notes:
- N/A