Support Redis Unix sockets

[minijackson]

Fixes: #15962

Add a URL field for the REDIS["tasks"] and REDIS["caching"] configuration settings, which takes priority over REDIS["tasks"]["HOST"], REDIS["tasks"]["PORT"], and so on.

I added the doc recommending the URL usage for Unix sockets, but it's also possible to recommend URL for all usages, since you can also specify redis:// / rediss:// URLs.

I haven't tested this using ./manage.py test yet, I plan to do that soon. This setup has been in use in the NixOS distribution since while back, without any issue.

[jeremystretch]

Thanks @minijackson!

[tacerus]

Thank you!