Make CAS login default on login forms

If by "default" you mean forcing a user to login via CAS when they visit /user/login, you can achieve this by adding user/login as a forced login path in the CAS settings. You can do the same thing if you want to set some other arbitrary path to force a CAS login.

We made a deliberate decision in the D8 port to NOT make any major modifications to the standard user login form. Instead we just provide an option to include a link on the login form to authenticate using CAS. Site themers can modify the user login form to style it as they'd like,