Migrate attachment associations to columns on attachments
Add pending migration to drop attachment join model tables
Remove old attachment clean-up scripts
Remove old attachment join models

We've replaced all five of the separate join models (e.g. EditionAttachment
and SupportingPageAttachment) with a polymorphic association.
DRY up keep/remove/replace attachment handling

The code that initiated the behaviour in Admin::AttachmentActionParamHandler
was duplicated across three controllers, making it easy to fail to
update one of them when the behaviour changed. This commit dries it up,
so any controller that includes the module only needs to call it in a
before filter.
Don't use send unnecessarily
simplify test data creation
Merge pull request #870 from alphagov/polymorphic-attachments

Tracker: https://www.pivotaltracker.com/story/show/57303956

Polymorphic attachments