Figure out how to deal with device names and keep them in sync with Synapse

Even in MAS mode, users can view and edit device names through the Synapse client-server API. (Maybe the device can only edit its own name, but it doesn't matter on the exact details.)

That said, we'd like to show device names in the MAS device management UI (because this makes each device more relatable).

However, we can't simply remove them from Synapse, because devices are the individual units acting as cryptographic parties, so clients have to have a client-server API to inspect those.

This seems to suggest that we need to keep device names in sync between MAS and Synapse, though maybe name changes can be funnelled through MAS.

Specific points to be addressed:

* [x] we ignore the initial display name clients send us on /login, we should probably still get those and record them
* [x] we should compute a human-readable device name from MAS and give that to Synapse when we provision them — this is tricky because it depends on the user locale
* [x] we should have a feature to let users rename their sessions in MAS
* [ ] we should catch device renames from Synapse and get them into MAS