Make it possible to have ordered collections (lists)

Now that we have public collections, it would be nice if it was possible for people to do stuff like "My best of 2012" or "Best selling releases of X week" or "Pitchfork's top 100 of the 1990s" or... well, you get the idea. This would need:

Some kind of ordering interface (which we could probably more or less reuse from ordered series once that's implemented).
Either a flag (ordered/unordered) so that order stuff isn't loaded where there's none, or just hide it if all position values are null.
A column "position" in the editor_collection_release table.