Fix Formation walking / other orders with the new UnitMotion
This fixes a number of issues with formations:
- Gives the controller an obstruction. This fixes the bug where units in formations can't gather from a tree.
- Stop special-casing formation members in PathResult. This fixes formation members being stuck in-place when they run in an obstruction.
- Makes sure units stay in Formation.IDle when they are idle so that D1337/D1901 can work correctly in the IDLE state.
- Warn if animals enter this state.
- Make sure that formation members that end up in INDIVIDUAL.IDLE go back to FORMATION.IDLE for sanity and for better housekeeping (refs #3144 - fixed completely upstream).
Differential Revision: https://code.wildfiregames.com/D2048