Added pghero and authorization for its view

We cannot use helpers in initialisers, so only access from auth_by_session is supported.
Closes #176

Downside from using the gem is, that auth check needs to be done in routes (don't match the route unless authenticated) and thus we needed to add a initialiser to use cancan in routes...
Added option to site.yml to disable/enable pghero.

/pghero will give 500, if not specified, but otherwise doesn't effect.