Rewrite/restructure gamesetup

[leper]

Currently gamesetup.js is not really extensible to support trigger/campaign maps where some settings should be fixed while others can be changed (e.g. allowing the player to assign himself to slots 1 and 3 on a 4 player map). The logic to decide what settings can be changed (for RMS, Skirmish Maps, and Scenarios) is also quite duplicated.

The code could also use some improvement such that the persistent game settings can work with just storing the minimum of information needed to recreate the settings instead of saving everything and then hoping that it doesn't cause conflicts/issues later (see #3044, #3033).