Allow passing pd & slack secrets via env var

In some deployments there are existing mechanism for handling secrets which would be preferred to having them in the mongo db.

This also unifies 2 code paths for configuring PagerDuty.

Testing:

• Automated tests pass
• Original flow of storing these secrets in mongo works
• Using env vars instead works (although the admin will incorrectly show an error for the slack token and I had to make any dummy edit to cause it to save the bot settings)