feat: Email signature API support (#3874)

db/migrate/20220131081750_add_email_signature_to_user.rb

@@ -0,0 +1,8 @@
+class AddEmailSignatureToUser < ActiveRecord::Migration[6.1]
+  def change
+    change_table :users, bulk: true do |t|
+      t.boolean :message_signature_enabled, null: false, default: false
+      t.text :message_signature, null: true
+    end
+  end
+end