Kill and Assassination tweaks, and small Monster tweaks

- Leveling up is done *before* capping and reacting to
  changes in stats instead of after now, and removed the
  max health recalc from the level-up script since the
  Vitality change check handles that for us now.
- Kill and Assassination missions will now grab a list
  of potential targets by their species-designated
  threat% based on the mission difficulty, so harder
  missions will now happily send you after bosses while
  easy missions will choose cannon fodder targets
  instead, with normal ones in between.