Nuclear Dawn 6.8a

Skirmish
- Added server cvar nd_skirmish_advkits to control whether or not Advanced Kits are enabled in Skirmish, default on.
- Updated loading screen text for Skirmish maps.
- Fixed spawn times getting hidden increase for each teammate killed while dead.
- Fixed loadout changes not pausing spawn timer.

Other fixes
- Added in-game browser fallback for functionality that requires showing a webpage when Steam Overlay is disabled.

Metro (Skirmish)
- Relocated some spawnpoints to avoid players getting stuck in t-pose.
- Added more spawnpoints for both factions.
- Updated sdk_content sources for sk_metro and added missing instances.