Repairable bridges

Putting this up for review now, but I plan on doing the rules for RA before merging.

Engineers get a repair cursor over the ends of a dead bridge, and you can force-attack to repair damaged bridges (to avoid conflict with the move order). Undamaged bridges will give a repair-blocked cursor.

Long bridges will grow from the repaired end at ~2 spans / second, and destroy any ships in its path.