Add diminishing Returns to several skills

References https://github.com/HabitRPG/habitrpg/issues/4419, https://github.com/HabitRPG/habitrpg/issues/4417, https://github.com/HabitRPG/habitrpg/issues/4416, https://github.com/HabitRPG/habitrpg/issues/4415, https://github.com/HabitRPG/habitrpg/issues/4414, https://github.com/HabitRPG/habitrpg/issues/4412.

I tried to keep values in line with what was there previously or in the notes. However, the diminishingReturns function generally buffs at low levels and nerfs at high levels compared to a linear function.