Add a customizable voting system

[0xlord]

For an unguided public server usage it would be enormous helpful to have a customizable voting system.
Currently I am hard setting the factions for the players and the enemies (as described in #454). But it would be great to have this customizable through the players via a voting. However this would require the overhaul which you mentioned in the ticket above earlier.

I think it would needed to be customizable so people can't vote for stuff which isn't loaded. For example you could have a hpp file in which you put in a list of factions which are voteable. If this is set people will then presented with a menu in which they can vote factions from this list. Is everything set, the mission will load with these factions loaded. If the hpp file is empty (or a parameter to have this voting not set) then the mission will load with the preset settings as it does now.