Support groups (private channels and group DMs) in the Slack adapter

[vanstee]

After rewriting the slack adapter with the refactored adapter protocol, it looks like we missed private channels (probably because we didn't have an integration test running against them). Let's add back support for them. As far as I can tell, all we need to do is accept both channel and group ids where we are currently only match on channel ids.

Thanks for the heads up @pygillier.