Add moderation note (#5240)

* Add moderation note

* Add frozen_string_literal

* Make rspec pass

app/models/account_moderation_note.rb

@@ -0,0 +1,22 @@
+# frozen_string_literal: true
+
+# == Schema Information
+#
+# Table name: account_moderation_notes
+#
+#  id                :integer          not null, primary key
+#  content           :text             not null
+#  account_id        :integer
+#  target_account_id :integer
+#  created_at        :datetime         not null
+#  updated_at        :datetime         not null
+#
+
+class AccountModerationNote < ApplicationRecord
+  belongs_to :account
+  belongs_to :target_account, class_name: 'Account'
+
+  scope :latest, -> { reorder('created_at DESC') }
+
+  validates :content, presence: true, length: { maximum: 500 }
+end