* Make `UnregisterProject` a request. This way the client-side project can wait
  to clear out its remote id until the request has completed, so that the
  contacts panel can avoid showing duplicate private/public projects in the
  brief time after unregistering a project, before the next UpdateCollaborators
  message is received.
* Remove the `RegisterWorktree` and `UnregisterWorktree` methods and replace
  them with a single `UpdateProject` method that idempotently updates the
  Project's list of worktrees.