Recalculate collection extents on item updates / deletes

[jisantuc]

Improvement

#623 handles growing collection extents for item changes. This is the easy version -- get the new extent represented by any changed items, union it with the existing extent, go home. However, in the case where items are deleted, or their extents are smaller, updating the collection extent requires re-evaluating the combined extent of every item in the collection. Probably that's reasonably fast with geom indices from PostGIS, but it should still probably be moved to a background task so that users don't
have to wait for us to recalculate collection extents when they make an item slightly smaller.