Add #recordings to ServersController
Review the docs to include information about recordings
Add #server to BigbluebuttonRecording

The server where the recording was fetched from.
There's also BigbluebuttonRecording#room.server, but since a room can change the server it is in, this shouldn't be used.
Fix errors synchronizing recordings and listing recordings of a server
Use a transacation in BigbluebuttonRecording#sync for each recording

If anything in a recording fails, the entire recording is not saved, but the other recordings are.