feature: custom game modes choose their Valve game mode and apply their cvars on community servers

[lukepolo]

Community servers running a custom game mode always booted on Valve's stock Custom mode and never ran the mode's cvars, so plugins like Deathmatch spawned everyone at team spawns.

• New game_modes.valve_mode picks the CS2 game mode a community server boots on (Custom when unset); the Deathmatch starter uses Valve Deathmatch
• game_type/game_mode values follow CS2's gamemodes.txt; Retake servers now boot CS2's retakes mode instead of CS:GO's skirmish
• A mode's cvars are written to the gamemode_<mode>_server.cfg CS2 runs after the Valve mode's own config on every map load
• Deathmatch, Retakes, 1v1 Arenas and Chaos are offered once as templates and stay gone when deleted
• Utility Practice is an official mode the utility system relies on, so it can no longer be deleted, archived, disabled or renamed