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/models/account.rb

@@ -155,6 +155,14 @@ class Account < ApplicationRecord
     ResolveAccountService.new.call(acct)
   end
 
+  def silence!
+    update!(silenced: true)
+  end
+
+  def unsilence!
+    update!(silenced: false)
+  end
+
   def suspend!
     transaction do
       user&.disable! if local?