Add initial placeholder view for custom domains

Author: simonbaird

rails/app/views/sites/_custom_domain.html.haml

@@ -0,0 +1,15 @@
+-#
+  In future this will be a form (or maybe a whole separate
+  page). For now it's read only.
+
+- if feature_enabled?(:custom_domains)
+  = render layout: 'field_wrapper' do
+    .form-text
+      %h5.mt-4 Custom Domain
+  .ms-3
+    = render layout: 'field_wrapper' do
+      = f.label :domain, 'Domain:'
+      = site.custom_domain&.domain || 'none'
+    = render layout: 'field_wrapper' do
+      = f.label :active, 'Status:'
+      = site.custom_domain&.status

rails/app/views/sites/_form.html.haml

@@ -52,6 +52,7 @@
       = render 'site_common/allow_in_iframe', f:
       = render 'save_method_prefs', f:
       = render 'skip_etag_check', f:
+      = render 'custom_domain', site: @site, f:
 
     = render layout: 'field_wrapper', locals: { pt: 'pt-5' } do
       = f.submit @site.new_record? ? 'Create' : 'Update', class: 'btn btn-primary'