Fixed rollbar integration if ROLLBAR_TOKEN is provided

[snipe]

This is a possible improvement/fix for #11989.

[what-the-diff]

• The return statement was changed to a variable assignment
• A new channel called 'rollbar' was added, which uses the Rollbar monolog handler and sets its level based on an environment variable (ROLLBAR_LEVEL)
• If there is a ROLLBAR_TOKEN in the .env file, then we add rollbar as one of our channels for stack logging

[jerm]

Way better! It sends rollbars when there's a token, and doesn't break when there isn't one! :-D

[snipe]

👍