List all document series grouped by orgs

Might mean chores like the following can ve avoided:
https://www.pivotaltracker.com/story/show/43754949
sort by org name
also sort by name
Add test for admin document series index page
Merge pull request #252 from alphagov/list-all-document-series

List all document series

https://www.pivotaltracker.com/story/show/43754949