EW: Use 3PID capability to show / hide email UI in settings

[Johennes]

There is a capability that instructs clients whether or not they can manage 3PIDs: https://spec.matrix.org/v1.7/client-server-api/#m3pid_changes-capability

@kerryarchibald notes that currently:

We just let the requests fail when the server doesnt support in EW I think, I vaguely remember talking to Travis about this. He suggested we should remove it completely 🤷

When the home server is connected to a MAS deployment, emails will need to be set on MAS rather than in EW.

We'll need to get this into the EW RC currently scheduled for August 22nd.

AC

• When the capability is off:
  • The email fields in settings are not editable
  • There is a link to the email / profile management page on MAS
• When the capability is on:
  • Things are as they are today

Note that this has an accompanying Synapse part: matrix-org/synapse#16128

[hughns]

FTR, this is now listed as part of MSC3824 Definition of OIDC aware clients