Consolidate all the login buttons into one

[FrostyX]

As per mailing list discussion

Thread:
https://lists.fedoraproject.org/archives/list/[email protected]/thread/RIS7TMRMVANSNGMPQHBUX6AOT7NOMZ4R/

And especially nirik's comment:
https://lists.fedoraproject.org/archives/list/[email protected]/message/S4AD2WDUVXQFIFKTHPPYYK2AJDBPG52T/

[praiskup]

The GSSAPI button saved me many times in history (when normal OID login was broken).

I'd prefer if we could keep that one, unless there are really strong reasons to drop it.

[FrostyX]

My thinking was ... if we merge them all together, we can first check if the user has a valid Kerberos ticket. If yes, use it. If no, redirect to OIDC login. But I didn't see the code in a long time so I am not sure how doable it is