Implement multitenant admin

[nemesifier]

The admin does not implement multitenancy, non super-admin users of different organizations can see/edit the content of other organizations.

• look at how it's implemented in openwisp-controller: https://github.com/openwisp/openwisp-controller/blob/master/openwisp_controller/config/admin.py (look for Multitenant prefix in the code)
• write some basic tests, see how they are written in openwisp-controller: https://github.com/openwisp/openwisp-controller/blob/master/openwisp_controller/config/tests/test_admin.py (look for the word multitenancy