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

app/views/auth/shared/_links.html.haml

@@ -3,7 +3,7 @@
     %li= link_to t('settings.account_settings'), edit_user_registration_path
   - else
     - if controller_name != 'sessions'
-      %li= link_to t('auth.login'), new_user_session_path
+      %li= link_to_login t('auth.login')
 
     - if controller_name != 'registrations'
       %li= link_to t('auth.register'), available_sign_up_path