found out the difference between spell disruption checks and concentration

renamed the disruption function and fixed the comments accordingly
fixing concentration remains a TODO
CheckSpellDisruption: only consider the cc bonus when the feat is there
CheckSpellDisruption: use GetSkill, so also the stat bonus is applied
partly reverted old 3f9adcb4: don't mix disruption and concentration

once we're in that branch, we already know the checks failed, so
don't give them another chance with a different calculation
implemented proper concentration checks