Allow spectators to place beacons, visible to other spectators.

[SoScared]

Spectators in-game love to interact and comment. Spectator beacons would be helpful for the crowd to quickly draw attention to interesting events!

To prevent annoyance, this feature ought to depend on an implemented mute feature that includes the spectator beacon: #34

Relevant OpenRA PR and issue ticket: OpenRA/OpenRA#13600, OpenRA/OpenRA#10390