* Add PerformInvite and refactor how errors get handled
- Rename `JoinError` to `PerformError`
- Remove `error` from the API function signature entirely. This forces
errors to be bundled into `PerformError` which makes it easier for callers
to detect and handle errors. On network errors, HTTP clients will make a
`PerformError`.
* Unbreak everything; thanks Go!
* Send back JSONResponse according to the PerformError
* Update federation invite code too
