add an invite a player label to the new game form
use hiccup form-to macro instead of normal vector elements
rename game-invitations column names

We don't decide colors yet and need to know who invited who in order to
see who has to accept the invitation later.
add an invited-by method for game invitations
create a game invitation instead of a game

this will later allow for confirmation
show a list of pending invitations on the new game page