Support Token-based registration

[shing-yu]

Two years ago, synapse already implemented token-based registration.
But now, the Element Android app (including Element X) still does not support this registration method.
Trying to register on a server that uses it will display "Your server needs to be upgraded."

I think this is an important method of registration, Element X should support it.

[ShadowRZ]

@shing-yu By now a Matrix server must uses Matrix Authentication Service so Element X can register on the server, and I think this is likely the case from now on (so will never support other register methods)

Note that in MAS scope, support for registeration tokens in itself (MAS) is being worked on.

[spaetz]

And I am very happy that a released version of MAS is supporting registration tokens now. But I believe the registration support in EXA in general is still very much at the band-aid level.