Make single policy page H1 singular.
Permit additional attachments as per 50063707.
Apply pending migration cleanups

The production code no longer refers to these tables, so these migrations are now safe to run
Merge pull request #846 from alphagov/policy-show-pages-h1-should-read-policy-not-policies

Make single policy page H1 singular.

https://www.pivotaltracker.com/story/show/56794106
Merge pull request #852 from alphagov/permit-more-attachment-types

Permit additional attachments as per 50063707.

https://www.pivotaltracker.com/story/show/50063707
Merge pull request #851 from alphagov/apply-pending-migration-cleanups

Apply pending migration cleanups