Implement login with email

[PiotrKozimor]

Support for m.id.thirdparty identifier type and medium field in identifier.
Specs:

• https://spec.matrix.org/unstable/client-server-api/#post_matrixclientr0login
• https://spec.matrix.org/unstable/client-server-api/#identifier-types

Sytest:
Can login with 3pid and password using m.login.password

Questions:

• How do we pass configuration? I suggest email_login field in client_api.
• Shall Dendrite send verification emails on its own before enabling this feature?

[kegsay]

:S please don't make multiple issues for one issue - move it to #1298

How do we pass configuration? I suggest email_login field in client_api.

sounds good.

Shall Dendrite send verification emails on its own before enabling this feature?

To some degree it will have to send emails itself I think, what does Synapse do?