CAS should use the public_baseurl instead of a separate cas_service_url

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.