db: enforce unique username across admins and players
db: enforce unique username across admins and players

To support single login page, we will have to make sure
that usernames for admins and players are unique across
two tables. So do not let anyone create an account or
player with username that already exists in either table.

Signed-off-by: Ilija Hadzic <47a09d4a4430975766160361f65fc72633be50b6@gmail.com>

ihadzic/jim,ihadzic/jim,ihadzic/jim