Add Slack dependencies and minimum boilerplate to our flask app

Add Slack dependencies and minimum boilerplate to our flask app

This entirely replaces our flask setup, (which also breaks our current
WSGI setup too) but is based on the exact demo code from:

https://github.com/slackapi/python-slack-events-api/blob/main/example/example.py

I want to start with this to ensure I can at least get a correct
challenge working. After I get this working this way, I'll plan to put
direct flask code back in place and integrated with the web server via
WSGI.