Removes device_id and ClientInfo
device_id is never actually written, and the matrix.org DB has no
non-null entries for it. Right now, it's just cluttering up code.
This doesn't remove the columns from the database, because that's
fiddly.
* Merge LoginHandler -> AuthHandler
* Add a bunch of documentation
* Improve some naming
* Remove unused branches
I will start merging the actual logic of the two handlers shortly