Nickserv

This branch adds nickserv. It is based on db-services branch.
If we're going to allow this new command, we should add a check to make sure the nick being added is not already in use by another user, and if it is, we should disallow the addition.  This prevents users from booting other users off of perfectly good, serviceable nicks that they're already using.

Additionally, as an extra protection against abuse of this command, it should run nicknames through `isValidNick` from txircd.utils to 1. prevent users from filling up their allowed nickname pool with invalid nicknames and 2. prevent users from adding potential UUIDs, since allowing UUIDs to be added to the reserved nick database could play erratically with the UUID fallback mechanism.