Search improvements

Search was much improved in 0.6 but there are still things to add:

- [x] Sorting search results
- [x] Investigate Solr highlighting
- [x] Multi-faceting
- [ ] Template tag for injecting search form
- [ ] Allow category and offer pages to use Solr to generate results (and so facets can be shown)
- [ ] Use edismax

This ticket is a rewritten version of #423 
Might be worth a comment say that this class isn't intended to be used as a view mixin even though it subclasses a view mixin. It confused me for a few seconds.