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

app/policies/account_policy.rb

@@ -9,6 +9,10 @@ class AccountPolicy < ApplicationPolicy
     staff?
   end
 
+  def warn?
+    staff? && !record.user&.staff?
+  end
+
   def suspend?
     staff? && !record.user&.staff?
   end