Make reordering subsections logic consistent
Make reordering subsections logic consistent

Make reordering subsections consistent with how announcements and timeline entries are reordered.
The code is still in a separate method, but the logic for the reordering is the same.

Also removes the `current_positions` method that doesn't seem to be used anywhere.

alphagov/collections-publisher,alphagov/collections-publisher,alphagov/collections-publisher