Admission-based registrations mode (#10250)

Fix #6856
Fix #6951

app/helpers/home_helper.rb

@@ -64,4 +64,14 @@ module HomeHelper
       content_tag(:div, &block)
     end
   end
+
+  def sign_up_message
+    if closed_registrations?
+      t('auth.registration_closed', instance: site_hostname)
+    elsif open_registrations?
+      t('auth.register')
+    elsif approved_registrations?
+      t('auth.apply_for_account')
+    end
+  end
 end