[Bug 997345] Initial take on using django_filter for search

As per discussion with Giorgos, we opted to solve this bug through multiple commits.  Starting at this one is the use of django_filter in the search page, this will enable us to add new filters easily in the next steps.
Since we're going to experiment a lot with the new `betasearch` view, feel free to do things in a different way. 

The `include non vouched` filter we currently have doesn't make a lot of sense. I would like to be able to `include all`, `only non-vouched` and `only vouched` profiles. 

Go crazy and try different things, suggest new interfaces and actions.