I should be able to keep nicknames / notes about other users

Summary: While Querki doesn't have as much of the "impenetrable handle" problem as LJ or DW, it does have at least some of that. So it would probably be worthwhile for me to be able to record how I know this Person.

Not specifically requested by Eric, but grew out of a conversation with him, July 23, 2019. He has multiple Users with the same Display Name, and is looking for ways to keep them straight. Once possibility there is for the Manager to be able to see a User's activity within the Space, which can help a lot, but having to do that over and over is a hassle.

So the idea here is that, when I see another Person within the Space (primarily in Comments and Sharing, but the concept here is a general one), it should work somewhat like it does in LJ/DW. I should be able to record a nickname for that Person, and when I see that Person subsequently, I should be able to easily get to that nickname.

Conceptually, this is pretty general. We might want to keep it to "nickname" for simplicity's sake, but in principle we could keep arbitrary amounts of information.

Implementation is interesting. This should not be data on the Space, it should be on the viewing User; it probably belongs in UserSpaceSession. And it actually adheres to the Identity being viewed, not the Person -- it should apply across Spaces. (Identities are public information, so this is reasonable.)