Use a thread for creating the Google Sheet when a new channel is created

Use a thread for creating the Google Sheet when a new channel is created

This allows us to return quickly so that Slack doesn't retry sending the event.

As implemented in this commit, this code is a bit cheesy in that it
disables the debug mode and error handling in slack_send_message,
(since the prior implementation of that depended on having a current
Flask app, but now that we are calling this function from a thread we
don't have a current app context).