Automate adding the bot user to new/included channels

[dishonesthips]

Bots can't invite themselves to channels 😞. We should use the user_token to invite the bot user to new/included channels so that it doesn't need to be done manually.

[dishonesthips]

Slack's Conversations API allows us to invite users to channels given their ID. Unfortunately, the API doesn't make it easy to retrieve bot IDs. We'll need to paginate through the users.list endpoint and look for the bot user.

In fact, this process could happen on startup and then be loaded into the GenServer state

[dishonesthips]

discovered: the auth.test endpoint to allow a bot to get its own id

This means we dont have to paginate through all users to get the id