Improve behaviour of formations stuck within other units.

Large units risk being stuck between other units. This is true in general, but particularly weird with formations, since individual units may well not be stuck, only the invisible formation controller.
This alleviates the issue by ordering units to move individually when the controller appears stuck.

It introduces a new "VERY_OBSTRUCTED" unit motion message, which triggers when a unit has failed to move for several turns.

Reported By: Angen

Reviewed By: Freagarach

Fixes #4935

Differential Revision: https://code.wildfiregames.com/D3209