Configurable access control

[baudhuina]

Rationale for this pull request

As of v1.2.1:

• clearcache is only accessible to superuser. We would like some regular users with extended privileges (sysadmins) to be able to clear caches in an operational environment.
• clearcache is visible on the Admin homepage of any user with access to Admin, even non superusers, which can attempt a clear cache, before being forbidden by the clearcache view.

Both issue are nicely solved by using a specific permission (clearcache.use_clearcache), to control the access to both the service and the display its section in the Admin's homepage.

Testing for the permission in the existing test_func() of the view preserves the current behaviour as long as the permission is not granted to anyone, since superusers have all permissions.

Changes summary

• Added model “CustomPermissions” (without databasse tables) to create clearcache.use_clearcache permission, and made migration.
• Changed ClearCacheAdminView.test_func() to test for the permission (this is backward compatible with the previous test of “is_superuser”, since superusers have all permissions).
• Now test for the permission in admin/index.html template, to avoid displaying the clearcache section on Admin’s homepage if the user is not authorized.
• Added tests to check permissions with regular user:
  • Created tests folder.
  • Original tests unchanged to assert backward compatibility, as test_superuser.py. Just moved CACHE_EXPIRE_IN_SEC constant at module level to avoid PEP8 violation (all-caps constants not supposed to be in functions).
  • Additional test as test_regular_user.py.
• Added python 3.11 and django 4.0, 4.1, 4.2 to test matrix.
  • Since Django 4.x does not support Python 3.7, split the only job of the workflow in 2 jobs (one for python python3.7, one for all other python versions) and issued a different tox command in each one.
  • Added pytest and pytest-django in dependencies in tox.ini as required by “Missing dependency” tox warning.
  • Upgraded workflow actions to v3 (checkout and python setup). v2 runs on node12 which is deprecated.
  • Added workflow_dispatch: event to be able to manually trigger the workflow. It could be removed, but I guess it doesn't hurt.
  • Slightly updated documentation to reflect the improved access control.

All tests are ok.