Multimedia case properties

This branch adds support for adding MM form properties to a case (part 1) as well as adding MM case properties to case detail lists (part 2).
https://confluence.dimagi.com/pages/viewpage.action?title=Multimedia+Case+Properties

May be easier to compare the two parts separately:
- Part 1: https://github.com/dimagi/commcare-hq/compare/master...9fd33d5
- Part 2: https://github.com/dimagi/commcare-hq/compare/18b04ba...537095b

You can actually use `self.build_version >= '2.6'` now; added that like a month ago, and it returns a `LooseVersion`