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/form/admin_suspension_confirmation.rb

@@ -1,7 +0,0 @@
-# frozen_string_literal: true
-
-class Form::AdminSuspensionConfirmation
-  include ActiveModel::Model
-
-  attr_accessor :acct, :report_id
-end