SSO registration/login flow is ugly and not mobile-friendly

[richvdh]

This is related to #9171, though focuses on the login flow rather than the UIA flow.

1. The first step depends on whether your client supports:

   • MSC2858-enhanced SSO login
   • SSO login, but not MSC2858
   • no SSO login at all

   The first instance is fine: your client links straight to the IdP's login page.

   If the client doesn't support SSO at all, (eg, very old element-android, element pre-december-2020), then you first get taken to a "login fallback" page which looks like:

   

   This is surprisingly not-awful. There is an issue about integrating it with the next IdP picker at Support picking an SSO provider during login fallback #9013.

   If the client supports SSO login but not MSC2858 (eg, element-web pre-~1.7.15, element-android pre-december-2020) then, after you click "Sign in with SSO", you get taken to a page which looks like:

   

   This is ugly, and not mobile-friendly.

2. Next, you are taken through the IdP provider's login flow.

3. After that, you get prompted to pick a (matrix) username:

   

   This is ugly, and not mobile-friendly.

4. After that, you are asked to give your consent for the matrix client to access your matrix account:

   

   That is ugly, and not mobile-friendly, and confusing as all hell to the user.

[richvdh]

fixed by #9277 etc