Stability

This new workshop statistic determines how balanced each settlement is
 * not having enough food, water, beds for settlers will decrease it (per settler)
 * smaller settlements (under 15 settlers) can never fully reach 100% stability
 * Happiness under 75% loses stability (even moreso under 65%, 55%, and 45%)
 * Wellness under 75% loses stability (even moreso under 60%, 50%, and 40%)
 * Stability stops dropping at 10% (which is really bad anyway)