Add rqcleanqueue to management commands

[rfreis]

Hi guys,
I hope this PR finds you all well!

During my daily usage on local environment faced situations that this command could help a lot - actually I'm already using it on my local environment - and thought could be a good idea share this with everybody else

If you find useful, I'll be happy to contribute with the required changes/upgrades to meet the project standards or further improvements

[selwin]

Hi @rfreis mind writing a unit test for this? I think it'd also be better to rename this to rq-empty-queue. RQ has clean_registries method, which does some house keeping tasks, so I don't want to use the clean terminology for something that will delete the entire queue.