Fix user activity sorting
Fix user activity sorting

Query the solr fields which are actually stored: For the global user
list, this is `order.user.activity`, for instance user lists, this is
`order.user.activity.instance_key`.

liqd/adhocracy,DanielNeugebauer/adhocracy,alkadis/vcv,phihag/adhocracy,alkadis/vcv,alkadis/vcv,alkadis/vcv,SysTheron/adhocracy,liqd/adhocracy,DanielNeugebauer/adhocracy,phihag/adhocracy,liqd/adhocracy,DanielNeugebauer/adhocracy,DanielNeugebauer/adhocracy,SysTheron/adhocracy,liqd/adhocracy,DanielNeugebauer/adhocracy,SysTheron/adhocracy,phihag/adhocracy,phihag/adhocracy,phihag/adhocracy,alkadis/vcv