Preventing Users from disconnecting GitLab account from CAS

Description

This issues originates from the same source as this one. We're collaborating with a very large and strategic organization and this issue is feedback they had sent me as part of their evaluation of GitLab.

After configuring CAS for a GitLab instance we need to prevent users from disconnecting their account from the CAS provider. We also want to disable the social sign-in option and ensure the API is unavailable. Currently, if a user disconnects, the next time they try to access GitLab (and sign in with CAS, which is the only configured option), they get a 422.