refactor: extract instance_key only once
Allow to configure separate domains per instance

This allows to configure separate domains per instance in the Adhocracy
configuration.

Static assets (CSS, JS) are still loaded from the main domain.

Be aware that you need to configure DNS, webserver, SSL outside of
Adhocracy for each domain.
Fix instance_domains for relative URLs

When navigating from a specially defined instance URL to a different URL
with relative URL setting, we previously erroneously stayed in the local
URL. Now we force absolute URLs in that case.
Add note about user session loss on domain change

User sessions are tracked through cookies. As cookies are only valid for
the domain of the server sending the cookie, users lose their session
when switching from one instance to another if they have a different
domain.

This could be worked around by requesting some third-party cookies with
javascript. This might be implemented later.
Merge pull request #886 from nidico/feature-instance-domains

Separate domains per instance