* 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
* Pass join errors through internal API boundaries
Required for certain invite sytests. We will need to think of a
better way of handling this going forwards.
* Include m.room.avatar in stripped state; handle trailing slashes when GETing state events
* Update whitelist
* Update whitelist
* s/QueryBackfill/PerformBackfill/g
* OutputEvent now includes AddStateEvents which contain the full event of extra state events
* Only include adds not the current event
* Get adding state right
