Add moderation note (#5240)

* Add moderation note

* Add frozen_string_literal

* Make rspec pass

db/migrate/20171005102658_create_account_moderation_notes.rb

@@ -0,0 +1,12 @@
+class CreateAccountModerationNotes < ActiveRecord::Migration[5.1]
+  def change
+    create_table :account_moderation_notes do |t|
+      t.text :content, null: false
+      t.references :account
+      t.references :target_account
+
+      t.timestamps
+    end
+    add_foreign_key :account_moderation_notes, :accounts, column: :target_account_id
+  end
+end