Usernotes: Modifying and deleting existing usernotes

[eritbh]

There's no real way to change or delete a note in a Usernotes object right now*. without any form of unique identifier on usernotes; timestamps sort of work for that, but not really. We'll have to come up with an API to record changes to usernote objects and delete them.

^{* You can modify existing notes, technically, because Usernotes.get() returns references to the same note objects it holds, so any changes made there will be serialized out. But the array that's returned isn't the internal array, so deletion isn't possible - that's intentional until we consider the API for that more thoroughly.}