Add map beacons and radar pings
Add radar pings to BaseAttackNotifier and HarvesterAttackNotifier
Change spacebar key to move to the last visible radar ping location
Add beacon/radar ping placements for nukes/ion cannons/chronospheres/iron curtains.
Update CHANGELOG for map beacons/radar pings
Merge pull request #4682 from ScottNZ/beacon

closes #2320
closes #3767
closes #3810