Add page-exit warning when user edits a collection
Add page-exit warning when user edits a collection

This works with the new push/pop warnings, so that we return to a
sensible state (a previous warning, or none at all) when the user might
be editing other items on this page.

OpenTreeOfLife/opentree,OpenTreeOfLife/opentree,OpenTreeOfLife/opentree,OpenTreeOfLife/opentree,OpenTreeOfLife/opentree,OpenTreeOfLife/opentree