[FEATURE] Refactor SlackBot code into a Slack demo

[oshoma]

PROBLEM
The core Sherpa functionality is packaged as a Python library which other apps can include.
In contrast, the Slack Bot functionality in /src/apps/slackapp is really more of a demo of
Sherpa's capabilities now, and it is not part of the Sherpa package we publish to PyPi. It would be nice to keep Slack functionality, but it should not live in the /src directory any longer.

SOLUTION

• Move the Slack functionality from /src/apps/slackapp to /demo/slackbot
• Update the tests and makefile as needed to ensure it still works
• Edit /docs to clarify that the Slack feature is a demo, not part of the Sherpa library
  • About Sherpa/index.rst
  • How To/Slack Bot
• Update the README as needed