nginx: allow custom nginx config under sites-enabled

[missytake]

fix #562

[link2xt]

I don't like the idea of expanding the scope with the ability to host arbitrary static websites on the same machine, because then you may want to host non-static websites, it may interfere with certificate renewal if something fails etc. I also don't see where this is requested in the linked issue #562.

If someone wants to host other websites next to chatmail, then chatmail needs to be virtualized and a proxy in front of it can dispatch traffic to chatmail or to another website.

[missytake]

True, maybe #618 is the better path forward here. People who want to host non-static website can be referred to this PR for manual tinkering, but supporting this use case is probably a bad idea.