Attachment validates presence of attachable

Tests not passing.
Fix tests broken by attachment>attachable validation
Fix legitimate attachable validation fails
Data migration removes hanging attachments
Attachment factory doesn't set sequence

It no longer needs to, the model does this anyway.

Also dryed up the attachment factory a little.
Fix tests, and remove 'unwanted' one.

The removed test exposes a bug we have chosen to ignore for now
related to uploading multiple attachments at the same time as
creating the attachable.  This functionality is earmarked for
re-building anyway, so we're not going to spend the time fixing
the existing code.
Merge pull request #1133 from alphagov/attachments-must-have-attachables

Attachments must have attachables

https://www.pivotaltracker.com/story/show/61473136