Custom maps - more bug fixes

We are, hopefully, getting closer to eliminating all Custom maps bugs. Here's the new batch fixed in this update:

Teleporting units & gigantic defence lines

Upgrades no longer unlock units that are disabled in maps

Minimap now resizes properly and proportionally

No more multiple hosts in the game lobby

Air Transport is now available by default

Fixed cities without unit icons

Air Units have 'can take units' unchecked by default

Country flags are now properly resizing in game menus

'No open games' message now disappears when a new game is opened

Country-specific units no longer show up in other countries' city menus