Add slugs for document collection groups

We opted against a database field, because if the group is
renamed, the page will still load fine.

https://www.pivotaltracker.com/s/projects/367813/stories/56955348
Merge pull request #1072 from alphagov/56955348-add-slugs-for-document-collection-groups

Add slugs for document collection groups



We opted against a database field, because if the group is
renamed, the page will still load fine.

https://www.pivotaltracker.com/s/projects/367813/stories/56955348