Edition attachments workflow refactor

Tracker: https://www.pivotaltracker.com/story/show/53063091

This work is primarily to enable us to extend the level of control users have over things like grouping and ordering of attachments by pulling attachment management away from the main edition edit form. This not only simplifies the code, but also has the added advantage of allowing us to remove those dreaded functional test macros which take so long to run. Editing editions now gives you a two--tabbed interface, with attachments being managed on the second tab.

Note that attachments for supporting pages, corporate information pages and policy advisory groups are still managed using the existing workflow (and thus some of the macros remain). The same goes for consultation responses: consultation responses are still nested within the main consultation form and thus so are their attachments. We'd have to pull out consultation responses as a whole if we want to manage their attachments separately.

Apologies for the hokey commit history, neither me nor @gma have the energy to rebase things into better shape right now.