CAS should use the public_baseurl instead of a separate cas_service_url

[clokep]

When using CAS you currently need to define a cas_service_url in the homeserver configuration. This ends up being a duplicate of the public_baseurl configuration and should be fairly easy to get rid of.

Reasons for doing this:

• Not having duplicated config is good. 👍
• This will match what SAML requires.

We'll need to deprecate the old option before just outright removing it.