DRY up templates by re-using reference_fields partial
Add new attachment fields for House of Commons papers
Add House of Commons paper fields to publication attachments
Show HoC metadata on attachments tab
Attachment can list possible parliamentary sessions
Relax validation on HoC paper number

They're typically numbers, but can also be formatted 1234-i or 1234-ii.
Use pull down menu for parliamentary sessions
Remove tests that aren't likely to fail

These tests are really just testing with or not ActiveSupport's
`#present?` works. It's not logic that's likely to break, so we'll be
better off with a slightly faster test suite.
Use hyphens to format parliamentary sessions

There's even a style guide. See page 3...

http://www.parliament.uk/documents/commons-information-office/p12.pdf
Show HoC metadata on publication attachments
Allow parliamentary session to be unset
Allow HoC metadata on consultations
Support hoc_paper_number and parliamentary_session on AttachmentsPresenter
Merge pull request #685 from alphagov/hoc-papers

Tracker: https://www.pivotaltracker.com/s/projects/367813/stories/42624557

Add House of Commons metadata to attachments