Minor Collections rework

[Southclaws]

The basics for collections are in, but a few changes need to land before stable release:

Backend

• Collections should support the usual descriptions, rich-content and most importantly: hero/cover images.
• Visibility needs to be added so Collections can be treated as:
  • published: listed on the /c collections list page
  • unlisted: only visible on profile
  • draft: private - naming here is a little weird though so I'm not sure if we're abusing the "Visibility" type or if it needs an extra value ("private") or "draft" should be renamed to "private" as it's more flexible? Needs some thought.
• Collections should satisfy the Datagraph Item interface and be vector'd in the semdex - coordinates are an average of the contained items
• Do collections need URL slugs? /c/favourite-posts could be nicer, but I think it should use a Mark, like threads as there's a much higher likelihood of collisions with user-supplied slugs. Should not be unique in DB I don't think.

Frontend

• /c/ is already set up for a collections list page, needs to be built out.
  • Sorting: at first, non-algorithmic but in future, can borrow from the new feed algo, perhaps "recommended" sorting value
  • Filtering: maybe by tag or keyword search?
• /c/{id} page needs rework and ?edit state
• Layout options should be implemented too:
  • Card view
  • List view
  • Gallery view? for collections consisting of Pages with assets? Cover images? Not sure yet...

[Southclaws]

Mostly done, all left:

• edit mode for cover/etc
• datagraph
• visibility