HTML attachments

This branch converts HTML versions into HTML attachments, allowing multiple attachments per edition, re-ordering of HTML attachments, and attachment metadata.

This branch does *not* enable HTML attachments for other records which can have file attachments. This includes consultation responses, supporting pages, corporate information pages and policy groups.

Pre-merge tasks:

- [x] Rebase after #991 is merged and remove `html_version` call
- [x] Squash db migrations and wrap updates in a transaction
- [x] add test for `POST :create, html: 'true'`
- [x] fix commit message `s/resource/response/`
- [x] fix attachment ordering stuff (see last WIP commit)

Possible spin-off work:

- Allow HTML attachments on responses, supporting pages, etc
- Redirect to the publication when trying to view a deleted attachment

Tech debt work:

- Move `FileAttachment` specific code out of `Attachment`

https://www.pivotaltracker.com/story/show/53797983