This follows the same flows-based flow as regular registration, but as
the only implemented flow has no requirements, it auto-succeeds. In the
future, other flows (e.g. captcha) may be required, so clients should
treat this like the regular registration flow choices.
A couple of weird caveats:
* If we can't validate your macaroon, we fall back to checking that
your access token is in the DB, and ignoring the failure
* Even if we can validate your macaroon, we still have to hit the DB to
get the access token ID, which we pretend is a device ID all over the
codebase.
This mostly adds the interesting code, and points out the two pieces we
need to delete (and necessary conditions) in order to fix the above
caveats.
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.
