C3_mimi_414_admin_3.18

Issue #414 
@Rania-Abdel-Fattah  @ranaelnagar  @Ahmed-Atef  @ahmed93  @Ahmed-Moataz  please review
for scenerio review: 
run bundle install (because i've added new gem)
rake db:schema:load
rake db:seed
write in address of the url : localhost:3000/admin/login
username : admin@example.com
password : password
feel free to create/edit/delete records from the database but take care of the validations
for documentation review @Ahmed-Moataz  i didn't document the permit_params lines as it's obvious and it's not a method or action 
for testing review run rspec spec/models/admin_user_spec.rb