Require sign prefix on class signs

Due to BattleArena's selfish and obtrusive sign handling, it is not
possible to place signs where the first line of the sign contains
a name of a BattleArena class without BattleArena interfering. With
this and the fact that both MobArena and BattleArena have a default
class called Archer, MobArena users very frequently complain that
either "only the Archer sign activates" (because BattleArena steals
the sign) or "the only class that doesn't work is Archer" (because
BattleArena manipulates the contents of the sign).

This commit adds a requirement to the BattleArena class signs such
that the optional class sign prefix becomes mandatory. This cleaner
approach will prevent BattleArena's messages from getting too long,
and even changing. Alternatively, the check can be moved into the
BASignListener class and accompanied by an error message.