Squashed the following bugs:
- "ttt_slowmotion_enabled 0" does not remove it from the buy menu
- Slow motion is not cancelled on round end or round start, causes issues in edge cases
- The specified duration is longer than set

Changes:
- ttt_slowmotion_duration has a new default value of 3

- fix convars

- sandbox support
- new cooldown convar "ttt_slowmotion_cooldown"

Minor bugfixes