[bug] Group has not been registered with django-reversion

[nemesifier]

This shows up in instances installed via ansible-openwisp2 but can be replicated in the dev env.

How to replicate:

• add reversion to INSTALLED_APPS, install django-reversion if you don't have it installed in your virtualenv already, in openwisp-users is optional
• go to groups, open a group, (eg: administrator), make some changes, save, repeat the process again
• click on the "history" button in the upper right part of the page
• different revisions will appear, click on any

Expected outcome:

Page with the revision appears.

Actual outcome:

<class 'django.contrib.auth.models.Group'> has not been registered with django-reversion

[nemesifier]

Fun fact, trying to register Group manually yields the opposite exception:

>>> from reversion import revisions
>>> revisions.register(Group)
Traceback (most recent call last):
  File "<input>", line 1, in <module>
    revisions.register(Group)
  File "/opt/openwisp2/env/lib/python3.8/site-packages/reversion/revisions.py", line 406, in register
    return register(model)
  File "/opt/openwisp2/env/lib/python3.8/site-packages/reversion/revisions.py", line 373, in register
    raise RegistrationError("{model} has already been registered with django-reversion".format(
reversion.errors.RegistrationError: <class 'openwisp_users.models.Group'> has already been registered with django-reversion

[nemesifier]

Oh I see now, Group is being interpreted in two different ways:

• openwisp_users.models.Group: registered
• django.contrib.auth.models.Group: not registered