Change where a collection sits in the list

Moves the collection to another place in the list of collections. The order is one for the whole portal - the same order every person sees in the sidebar - so this changes it for everybody, and it takes the portal-wide right to create collections, exactly as the sidebar does. position is the sort weight the listings answer with: pass a number between the weights of two collections to land between them, and omit it to land last. The weight actually stored is recomputed from the neighbours - it may differ from the one asked for - so the answer carries the collection as it was saved. An archived collection is not reordered: it is out of the list to begin with.