Privileged User Unreads

[cohoe]

Should fix #216 by adding logic to make Redis notification data follow what we do in the DB for mod/tt unread messages.

Starts creating a central place to codify privileged user names since we have those strings scattered throughout the app and it's hard to remember what the values are supposed to be.

[challfry]

I may come back and tinker with this later; code that currently checks against specific privileged usernames almost always either wants the UserCache entries for those users or wants to do an authorization check. Both of these use cases may be better served by adding methods to PrivilegedUser, instead of having PrivilegedUser vend out names. But we're close to boat, so--this looks fine.