Edit all Instances should manage pagination with the scrollbars

Summary: Instead of old-fashioned coarse-grained pagination, Edit all Instances should simply manipulate the scrollbar. Ideally, the approximate length of the "page" should be calculated from the first ten instances; when you drag the scrollbar and land, it should load the live Editors for where it lands.

This is not easy to do well. But things like the Parallax-style viewers suggest that it is possible in modern browsers to manipulate the scrollbar to this degree.

Ideally, this should show a Loading icon until the new Editors get fetched.

This entry is here mainly so I don't lose track of it. It won't be soon, but it's the way things should work.