Allow bots to have their own inventories

[rivques]

This is mostly doable - we just need to adjust the logic in the trading api implementation

[rivques]

Right now this is possible, but hacky, and requires direct access to the DB. You have to go to the Slack bot's identity and add its Bag app id to its specific apps, which authorizes it to use its own user id as a source in makeOffer.

[rivques]

Really we should just do this automatically when you run /bot.