Solr-based milestone proposal pager

In order to allow different default sorting in the milestone proposal pager (#858, #860), it has to be backed by Solr. To do so, we need to do two extensions:

* Add delegateable *frozen* state to Solr. 
* Allow to add OR-filters to SolrPagers.

The former is straight-forward. I struggled for a while to create a nice API for the latter (i.e. extend `extra_filter` to allow for a more generic construct), but wasn't happy with the results and in the end decided to just do the minimum for what is required here. If we need more, we can extend it later.