Data migration for relevant_to_local_government

All policies that are local-gov specific, and all documents tagged to
those policies should then get the local gov flag checked.

Need to run: `rake db:data:migrate`

https://www.pivotaltracker.com/story/show/43862753
Fix rake task getting killed error

Use update_column instead of update_attribute
Also update the policy with relevant_to_local_government

This is not really possible to see on the admin frontend

Also iterate through all versions of an edition as there could be
drafts that also need updating
Policy: set can_apply_to_local_government to true

Allow setting of this flag in the Admin UI
Merge pull request #358 from alphagov/data-migration-update-local-gov-flag

Data migration for relevant_to_local_government

https://www.pivotaltracker.com/story/show/43862753