Move django-after-response to optional?

[gsilvapt]

In this snippet, there's this FIXME comment here but, however, in #24 I tried moving this dependency to optional and some tests immediately failed, which made me wonder whether this dependency is that optional or not.

django-dkron/setup.cfg

Line 33 in 21b48b4

# FIXME: remove this "feature" (dependency)? move it to optional?

We should track the work necessary for this comment in this issue because it's definitely not just about moving the dependency around and updating docs.

[fopina]

If it was just moving to optional it would have been done instead of a FIXME 🧌

I think it's an old note in a sense of open question, whether this after_response should be dropped and only support run_async with a running dkron (that is the code that uses it)