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/statuses/_status.html.haml

@@ -56,6 +56,6 @@
 
 - if include_threads && !embedded_view? && !user_signed_in?
   .entry{ class: entry_classes }
-    = link_to new_user_session_path, class: 'load-more load-gap' do
+    = link_to_login class: 'load-more load-gap' do
       = fa_icon 'comments'
       = t('statuses.sign_in_to_participate')