Add link to contact form to error page

[toothstone]

Since SIPA is now available for unauthorized users, we should link to our contact form there.

[fgrsnau]

Unfortunately, we would need some kind of custom message on the error page. This is currently not implemented. One could update the template /usr/share/hades/templates. Those changes are not permanent.

There multiple solutions:

1. Override the template via a file in /etc/hades/templates.
2. Fetch custom messages from a database.
3. Add a config option HADES_PORTAL_CUSTOM_{MESSAGE,FOOTER,WHATEVER}.

The last one is quite easy to implement and it is also the least flexible one. Making every aspect of hades fully customizable is not worth the overhead imho.

[toothstone]

To be honest, I would probably have just put it in the template base.html, since there are lots of hardcoded strings in there already. But an additional config option works for me just as well, I wasn't aware that the mail address is included this way.