Previously, we served all domains identically, but the Apache backend
redirected all but the main domain to the main domain (`www.`).
We now moved these (302) redirects up to nginx, which is faster, but
more importantly also covers requests not proxied to Apache.
As of now, this only concerns Hedgedoc. Our Hedgedoc setup does not
work well with non-main domains, which prompted this change.
Each "webapp" now also has its own `*_domain_names` variable.
88f090d5d5