Add region/category/carrier filtering on Collections (bug 903489)

Also, implement dynamic filters fallback : if the current set of
filters don't return results, remove one and try again till we have
exausted available filters or we have found some results.

django-filter 0.7 upgrade is necessary to fix a bug regarding filtering
on '0' values.
Merge pull request #981 from diox/collections-region-category-carrier

Add region/category/carrier filtering on Collections (bug 903489)