Refactor user management

[sloane-shark]

Maybe not on the scale we hoped last time we tried this (a single unified interface to access users by nick, real name, or firebase alias), but at the very least we need to redo /add and /kick to be more flexible and work like we expect. Fixing this should address #48, #46, and probably incorporate #32 since it will be related.

[sloane-shark]

To be more specific, we want to only add users who we expect (users who were previously removed from the chat, most likely). We should probably keep a list of userID's that gets updated on user join or user remove (manually or with /kick). We could always add an /addroulette command that brings back the fun of the old way.