Add in data migration to populate doc series summary

Expect ~ 986 to be updated

https://www.pivotaltracker.com/story/show/49582421
https://www.pivotaltracker.com/story/show/49119183
Merge pull request #547 from alphagov/invalid-document-series

Add in data migration to populate doc series summary

https://www.pivotaltracker.com/story/show/49582421
https://www.pivotaltracker.com/story/show/49119183