Fix race condition when server is only partway added

[EvanHahn]

Adding a server peer basically has 3 steps:

1. Tell the server to add itself to the project.
2. Assign a role to the server peer.
3. Wait for an initial sync with the server peer.

A bug can arise if steps 2 or 3 fail, or if step 1 succeeds on the server but fails on the client (likely due to network failure). If this happens, trying to re-add the server will fail because the server will respond with “you can’t do this, we already have this project”.

To fix this, we should:

• Make adding a project idempotent on the server side. (See the associated issue on comapeo-cloud.)
• Make assigning the role idempotent in this repo.