Skill rebalancing

Working on implementing skill changes discussed in https://github.com/HabitRPG/habitrpg/issues/3029

So far, this implements:
Backstab - New reward calculations, as proposed by @ShabbyX
Burst of Flames - Changed damage to 10% of INT, as proposed by @deilann
Tools of the Trade - Increased buff amount, as proposed by @Alys

@betaveros has his proposed fix for Valorous Presence in pull request #356