Admission-based registrations mode (#10250)

Fix #6856
Fix #6951

app/helpers/application_helper.rb

@@ -20,7 +20,23 @@ module ApplicationHelper
   end
 
   def open_registrations?
-    Setting.open_registrations
+    Setting.registrations_mode == 'open'
+  end
+
+  def approved_registrations?
+    Setting.registrations_mode == 'approved'
+  end
+
+  def closed_registrations?
+    Setting.registrations_mode == 'none'
+  end
+
+  def available_sign_up_path
+    if closed_registrations?
+      'https://joinmastodon.org/#getting-started'
+    else
+      new_user_registration_path
+    end
   end
 
   def open_deletion?