First pass at HTML versions of attachments

I've made a first pass: adding the data model and put up a simple display page.

Also refactored the attachment listing code into `AttachmentsPresenter` and added a simple `HtmlAttachment` adapter that quacks like a regular `Attachment` class for display purposes.

Note there's no preview functionality yet for HTML versions - you have to publish to see the page. Open to ideas for the best way of doing that: would a preview link above the text body be sufficient, or do we need to support special preview urls for these versions?

Assuming the code is good, this can be accepted as is, with the caveat that it allows writers to create HTML versions of their documents if they choose to put text into the relevant fields.

Ticket: https://www.pivotaltracker.com/story/show/45760867