Add power sabotage ability to spy

This adds a power sabotage ability to the ra and ts spies in an effort to make them more useful.
I haven't managed to find any suitable sounds I can mix, so there isn't much feedback to the player being infiltrated apart from a yellow selection bar and the disabled overlay for power plants.
I don't think PowerOutageModifier is a good class name, but I haven't been able to come up with something better.
I would like to kill off the float/decimal modifiers before we add new ones, to avoid writing things twice. See the discussion in #6079.