#1 Using username instead numeric value

[Teemu]

On classes.inc class table is associated with it's
creator by userID (username) field which opposes a
risk, it should be the auto_increment value from User
table instead.

If a user creates a class, then her account gets
removed (of course at this point the system should say
that there's relations left) and then someone gets the
same username (but is totally different person) she
will be listed as the owner of that class.