Write script to configure Slack App

[gregtyler]

When you run the code locally, you need to set up a separate Slack App with all the write permissions as described in slack_app_create and slack_app_config.

You also need to reconfigure some of these options when your Ngrok URL changes, which happens at least every 12 hours.

If supported by Slack, we should write a bash script that can configure the Slack App for you so you can run it both on initial setup and when your Ngrok URL changes.

If that's not supported by Slack, we should figure out a way of having a more stable URL so you don't have to reconfigure Slack every time.