Attachment order fixer & tests.

Includes test for the normal case.
Add attachment fixer test for bad created_at case
Add attachment fixer test for replaced attachments
Avoid reordering known manually ordered docs
Don't "fix" attachment ordering on new docs.
Skip docs which don't support attachments.
Don't use ID order of 1st eds after timestamp fix

The "first edition created after the polymorphic 
deploy" rule doesn't hold for documents which were
editioned after the fix which relied on attachment
created_at.

Technically this only applies to imported editions
but it's not worth pulling that apart to check.
Load attachment fixer fixtures from SQL files
Merge pull request #972 from alphagov/fix_attachment_ordering_part_3

Fix attachment ordering part 3

https://www.pivotaltracker.com/story/show/58806434