Google Code-in

Move openwisp_controller.admin.AlwaysHasChangedMixin to openwisp-utils (other modules are duplicating the same logic so we need to share the code to avoid duplication).

• open a pull request on openwisp-utils
• ensure the test coverage of openwisp-utils does not decrease please (you need to write test code)
• once merged, open a pull request on openwisp-controller to use the code which you just added to openwisp-utils

Don't forget to start and fork the github repositories linked in this task, this will help us for the reason described in our documentation page Help us to grow.