Add user notes on accounts (#14148)

* Add UserNote model

* Add UI for user notes

* Put comment in relationships entity

* Add API to create user notes

* Copy user notes to new account when receiving a Move activity

* Address some of the review remarks

* Replace modal by inline edition

* Please CodeClimate

* Button design changes

* Change design again

* Cancel note edition when pressing Escape

* Fixes

* Tweak design again

* Move “Add note” item, and allow users to add notes to themselves

* Rename UserNote into AccountNote, rename “comment” Relationship attribute to “note”

app/models/account_note.rb

@@ -0,0 +1,20 @@
+# frozen_string_literal: true
+# == Schema Information
+#
+# Table name: account_notes
+#
+#  id                :bigint(8)        not null, primary key
+#  account_id        :bigint(8)
+#  target_account_id :bigint(8)
+#  comment           :text             not null
+#  created_at        :datetime         not null
+#  updated_at        :datetime         not null
+#
+class AccountNote < ApplicationRecord
+  include RelationshipCacheable
+
+  belongs_to :account
+  belongs_to :target_account, class_name: 'Account'
+
+  validates :account_id, uniqueness: { scope: :target_account_id }
+end