track active players, use for team ballance

[justjake]

when a channel is enabled, track player activity in the channel with special bot actions:

PLAYER_DISCUSSED - when an enrolled player says something in the channel
PLAYER_COMMAND - when an enrolled player performs a command

tracking these events along with timestamps will allow us to track "active players", which should in turn allow the join command to intelligently select a team for a new player to join