Create device model and setup devise, activeadmin and rspec!

- with attributes: model, imei_number, unique_id and access_token
 - access_token is automatically generated on create

spec/models/admin_user_spec.rb

@@ -0,0 +1,4 @@
+require 'rails_helper'
+
+RSpec.describe AdminUser, type: :model do
+end