delegatebot should invite users to Slack channels

[dpb587]

When the bot mentions a user in a channel they’re not currently a part of, the user is not notified. Consider:

• automatically having the bot invite them (might not be desirable behavior for private channels)
• replying with an additional comment to the thread to let the user decide what to do and invite them

[s4heid]

This one might be a bit tricky.

There is a method InviteUserToChannel which calls the channels.invite endpoint. This endpoint requires a token of scope user, but the bot itself only has a bot scoped token which is not sufficient.