Custom homeserver on /login should use public-facing server name instead of delegation URL

[fnwbr]

Right now you have to provide the full URL to where the homeserver is living, such as synapse.example.com instead of example.com, or matrix.medienhaus.dev instead of medienhaus.dev.

This currently also results in the login form showing something like username:matrix.medienhaus.dev, when in fact the full user handle is username:medienhaus.dev.

Let's fix this by implementing a proper lookup of the potential .well-known delegation documented in https://matrix-org.github.io/synapse/latest/delegate.html. This is how Cinny is doing it: https://github.com/cinnyapp/cinny/blob/689adde8ae148d2de76bab0d11d7e0e8f35b7439/src/util/matrixUtil.js#L12C23-L12C33