Add moderation warnings (#9519)

* Add moderation warnings

Replace individual routes for disabling, silencing, and suspending
a user, as well as the report update route, with a unified account
action controller that allows you to select an action (none,
disable, silence, suspend) as well as whether it should generate an
e-mail notification with optional custom text. That notification,
with the optional custom text, is saved as a warning.

Additionally, there are warning presets you can configure to save
time when performing the above.

* Use Account#local_username_and_domain

spec/mailers/previews/user_mailer_preview.rb

@@ -39,4 +39,9 @@ class UserMailerPreview < ActionMailer::Preview
   def backup_ready
     UserMailer.backup_ready(User.first, Backup.first)
   end
+
+  # Preview this email at http://localhost:3000/rails/mailers/user_mailer/warning
+  def warning
+    UserMailer.warning(User.first, AccountWarning.new(text: '', action: :silence))
+  end
 end