Use CShowActivityEx & Fix Prefix consistency

Changed all CShowActivity to CShowActivityEx
Also changed [BOSS] in CShowAcitivtyEx to [BossSpawns] to keep the same as the ReplyToCommand which uses [BossSpawns] instead of [BOSS]

NOTE: This doesn't fix colors displaying in CShowActivity, the issue is with MoreColors, [This version](https://forums.alliedmods.net/showpost.php?p=2128587&postcount=390) however does work