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/sessions/new.html.haml

@@ -4,24 +4,25 @@
 - content_for :header_tags do
   = render partial: 'shared/og'
 
-= simple_form_for(resource, as: resource_name, url: session_path(resource_name)) do |f|
-  .fields-group
-    - if use_seamless_external_login?
-      = f.input :email, autofocus: true, wrapper: :with_label, label: t('simple_form.labels.defaults.username_or_email'), input_html: { 'aria-label' => t('simple_form.labels.defaults.username_or_email') }, hint: false
-    - else
-      = f.input :email, autofocus: true, wrapper: :with_label, label: t('simple_form.labels.defaults.email'), input_html: { 'aria-label' => t('simple_form.labels.defaults.email') }, hint: false
-  .fields-group
-    = f.input :password, wrapper: :with_label, label: t('simple_form.labels.defaults.password'), input_html: { 'aria-label' => t('simple_form.labels.defaults.password'), :autocomplete => 'off' }, hint: false
+- unless omniauth_only?
+  = simple_form_for(resource, as: resource_name, url: session_path(resource_name)) do |f|
+    .fields-group
+      - if use_seamless_external_login?
+        = f.input :email, autofocus: true, wrapper: :with_label, label: t('simple_form.labels.defaults.username_or_email'), input_html: { 'aria-label' => t('simple_form.labels.defaults.username_or_email') }, hint: false
+      - else
+        = f.input :email, autofocus: true, wrapper: :with_label, label: t('simple_form.labels.defaults.email'), input_html: { 'aria-label' => t('simple_form.labels.defaults.email') }, hint: false
+    .fields-group
+      = f.input :password, wrapper: :with_label, label: t('simple_form.labels.defaults.password'), input_html: { 'aria-label' => t('simple_form.labels.defaults.password'), :autocomplete => 'off' }, hint: false
 
-  .actions
-    = f.button :button, t('auth.login'), type: :submit
+    .actions
+      = f.button :button, t('auth.login'), type: :submit
 
 - if devise_mapping.omniauthable? and resource_class.omniauth_providers.any?
   .simple_form.alternative-login
-    %h4= t('auth.or_log_in_with')
+    %h4= omniauth_only? ? t('auth.log_in_with') : t('auth.or_log_in_with')
 
     .actions
       - resource_class.omniauth_providers.each do |provider|
-        = link_to t("auth.providers.#{provider}", default: provider.to_s.chomp("_oauth2").capitalize), omniauth_authorize_path(resource_name, provider), class: "button button-#{provider}", method: :post
+        = provider_sign_in_link(provider)
 
 .form-footer= render 'auth/shared/links'

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