Enhance experience when multiple authentication methods are available

[pmaier1]

Currently, the experience is good enough when:

• there is only one IDP and no password auth, because the upstream IDP login happens right away
• there is only password auth, because the user is presented with a username/login form

We need to enhance the experience when there are multiple upstream IdPs, and/or a mix of upstream IdPs and local password authentication options.

• Administrators can brand their IdPs (GitHub, Google, GitLab, Facebook, Apple) Administrators can give a human-readable name to their IdPs #2065
• Users can link/unlink multiple IdPs to their account
• Users can discover which IdP they used from their username (?)

[matrixbot]

For your information, this issue has been copied over to the Element fork of matrix-authentication-service: element-hq/matrix-authentication-service#1874