Refactor ProgressRecorder for non-optional Celery usage and add doc strings

See merge request !494

Rewrite dirtyfields with FieldTracker

See merge request !492

Release 2.0a4

See merge request !488

Resolve "[About] Shows versions as "unknown""

Closes #365

See merge request !487

Add django-extensions

See merge request !486

What did I drink when I wrote the original code? The relative import
`from .. import __version__` in the utility code in core, of course,
always imported from `aleksis.core`, no matter the real `AppConfig`
that inherited the utility code. Thus, the versions always showed the
version of AlekSIS-Core everywhere (whose version information was
broken ever since we renamed AlekSIS to AlekSIS-Core).

This change decouples the `get_version` code from the package level
`__version__` information, which effectively becomes optional now.

Rewrite logic for AlekSIS/Django user/group sync to only save if necessary

Closes #350

See merge request !485

Resolve "Relative imports fail when __main__ is not called correctly"

Closes #373

See merge request !484

Follow-up on 2f7ccc1d

Resolve "runserver stopped working with aleksis-admin in Django 3.1.6"

Closes #372

See merge request !483

> commit ec6d2531c59466924b645f314ac33f54470d7ac3
> Author: William Schwartz <wkschwartz@gmail.com>
> Date:   Mon Jan 4 08:50:49 2021 -0600
>
>     Fixed #32314 -- Fixed detection when started non-django modules with "python -m" in autoreloader.
>
>     django.utils.autoreload.get_child_arguments() detected when Python was
>     started with the `-m` option only for `django` module. This commit
>     changes the logic to check __spec__, see
>     https://docs.python.org/3/reference/import.html#main-spec
>
>     Now packages can implement their own __main__ with the runserver
>     command.

Thanks to William for breaking the API in a a.b.c.d.e patch-level
release !

Resolve "Reinvestigate how to do global permissions"

Closes #344

See merge request !453