Implement fully spec-compliant registration process

[druig]

Clients should not assume what registration process servers use and should use the user-interactive API to query flows and stages.
Fallback to web-based auth should be added first to comply with the spec.
Currently even the matrix.org instance doesn't handle fallback registration stage.
So we have to wait for matrix-org/synapse#4187

[hawkowl]

@druig Hi, the lack of support on matrix.org is due to a bug that slipped through in our Python 2->3 migration, and will be fixed shortly (that linked bug will track the solution).

[druig]

@hawkowl Looking forward to it!