Rename Waypoint -> Immobile and move it into Mods.RA.
Use Immobile for cameras and flares.
Don't render GravityBombs that are obscured by fog.
Rework airstrike flare and camera spawning.
Remove airstrike beacon when the attack begins.
Merge pull request #4853 from pchote/airstrike-fixup

Airstrike improvements