Resolve "Configurable login background"

That should close #327 (closed). Tested in docker. I've set up a new context processor with the variable, because there is no easy access to the login view (as it's hidden in django-two-factor-auth middleware).

I put WIP, so that you @piotr.gawron or @carlos.vega can review it.

Sorry it took so much time, I got stuck in the loop of burdensome troubles while testing. Note, that you might have had some dead code in login views (because the responsibility of logging in and out is now done via middleware).

PS. Would you like me to share the docker-compose code I used to test the project?