Accept and return slugs for category/region/carrier in collections API (bug 912279)
Merge pull request #1091 from diox/collections-return-slugs-for-stuff

Accept and return slugs for category/region/carrier in collections API (bug 912279)
Allow filtering on ids and on empty (null) values in Collections (bug 911382)

The empty/null part requires a custom implementation of FilterSet.qs,
because out of the box django-filter doesn't allow this at all.
Merge pull request #1082 from diox/collections-filtering-null

Allow filtering on ids and on empty (null) values in Collections (bug 911382)