Improve the structure of `Settings` in the hackbot-api service

[suhaibmujahid]

These settings live in services/hackbot-api/app/config.py and could be better organized. Specifically:

• Ensure that settings which should be mandatory are actually marked as mandatory.
• Clean up unnecessary comments, and condense overly long ones.
• Use consistent naming for webhook settings, ideally by nesting them under their respective platform's settings. For example, the Phabricator config should include its webhook config as well.