Add OMNIAUTH_ONLY environment variable to enforce externa log-in (#17288)

* Remove support for OAUTH_REDIRECT_AT_SIGN_IN

Fixes #15959

Introduced in #6540, OAUTH_REDIRECT_AT_SIGN_IN allowed skipping the log-in form
to instead redirect to the external OmniAuth login provider.

However, it did not prevent the log-in form on /about introduced by #10232 from
appearing, and completely broke with the introduction of #15228.

As I restoring that previous log-in flow without introducing a security
vulnerability may require extensive care and knowledge of how OmniAuth works,
this commit removes support for OAUTH_REDIRECT_AT_SIGN_IN instead for the time
being.

* Add OMNIAUTH_ONLY environment variable to enforce external log-in only

* Disable user registration when OMNIAUTH_ONLY is set to true

* Replace log-in links When OMNIAUTH_ONLY is set with exactly one OmniAuth provider

config/locales/en.yml

@@ -844,6 +844,7 @@ en:
     invalid_reset_password_token: Password reset token is invalid or expired. Please request a new one.
     link_to_otp: Enter a two-factor code from your phone or a recovery code
     link_to_webauth: Use your security key device
+    log_in_with: Log in with
     login: Log in
     logout: Logout
     migrate_account: Move to a different account