Make stubs singletons

This is a first step toward making the client-side object model stay in sync with the server.  Here it handles most object model changes when objects are deleted.

However, this may have performance ramifications, and may subtly affect mapping functionality, so should be tested, especially where it affects GDrive/GCal and permissions changes.  (I have not found any issues, but these are the special cases for stubs and uses of `Context` objects.)