Allow badges to impact mixed sorting (fixes #365) (NOTE: Reindex solr!)

This adds a new integer field to all badges. In special sorting methods
(currently only in the mixed proposal sorting method), this field is
used as the first level sort key, i.e. badges with higher impact value
are always put on top of badges with lower impact values.

At a later stage, a mixed sorting method for users and instances can
easily be implemented as well, e.g. using impact as first level sort key
and then activity.

This is implemented by normalizing the "base mixed score" to -pi/2 to
pi/2 using arctangent, and then adding (impact * pi). An alternative way
would be to add impact directly into solr and use multiple sort_by
methods in sequence.

NOTE: As the score calculation has changed, the solr index has to be rebuilt for all proposals:

    bin/paster index INDEX proposal -c etc/adhocracy.ini
    bin/paster index DROP proposal -c etc/adhocracy.ini