Roles and formats - potential generic solution

[andybak]

1. We set a boolean for "Displayable" and "Downloadable"
2. We have a field on the Asset that points to the preferred downloadable and preferred displayable.
3. We use the preferred displayable in the viewer
4. Downloads default to the preferred downloadable
5. We offer the other downloadables as optional formats on the download page
6. Maybe the viewer could also offer the option of switching to other displayables.
7. Existing legacy tilt roles are preserved but used only as extra metadata "hints" for users that want them

[jamesrampton]

@andybak for info, we are now displaying the same formats in the API as we are on the downloads page.