Make the bot appear online when available

[wbowling]

Currently ota_bot always appears offline, should be possible https://api.slack.com/docs/presence-and-status#bot_presence

[Kileak]

This doesn't seem to be easily fixed.

Since we're using a real user for the bot, he will go "away" 30 minutes after bot start.

Even subscribing the presence_change event and calling users.setActive for the bot user and
setPresence(presence="auto") when he goes away doesn't change his presence state back to online.