Move shared logic here

[nemesifier]

• admin theme
• DependencyLoader and relative tests
• DependencyFinder and relative tests
• TimeStampedEditableModel
• TimeReadonlyMixin, maybe we can rename this to TimeReadonlyAdminMixin
• MultitenantAdminMixin and relative tests
• MultitenantOrgFilter and relative tests
• MultitenantObjectFilter and relative tests

Tests

The tests will be the tricky part, we should ensure some test classes can be reused (the multitenant admin for example), so we move the main test logic to this module and in the other modules we just import the test classes and repeat them there to ensure the integration works correctly, there's an example of this technique in openwisp-controller, which imports the openwisp-users admin tests.

In the case of openwisp-controller, importing the admin tests from openwisp-users was necessary because an integration bug was not spotted during development until it went into production.