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_warning_preset.rb

@@ -0,0 +1,15 @@
+# frozen_string_literal: true
+
+# == Schema Information
+#
+# Table name: account_warning_presets
+#
+#  id         :bigint(8)        not null, primary key
+#  text       :text             default(""), not null
+#  created_at :datetime         not null
+#  updated_at :datetime         not null
+#
+
+class AccountWarningPreset < ApplicationRecord
+  validates :text, presence: true
+end