Migrate from $Stewardess to $StewardessID

After !6933 (merged) and !6935 (merged), there really weren't a lot of places left using $Stewardess, so I've taken advantage of that and just switched everything else over to use $StewardessID. One more subtask that can be checked off in #1717 (closed).

Also fixes the stewardess bonus to servant performance, which I had accidentally broken since it was relying on an unregistered global.

BC Required, of course.