Improve logging configuration and detail

[reefdog]

This is an umbrella issue for a handful of sub-tasks and for the tasks that fall in between them.

• Configure logger to do the right thing in test environments #87: While running tests, log to test.log file instead of console
• Improve PM2 logging configuration #233: Replace PM2 log symlinking with proper configuration
• Allow yarn tasks to log to stdout in production #248: Make sure we can inspect the app using yarn tasks in production (since currently, tasks like yarn queue:jobs:list aren't usable in production due to console output suppression)
• Dump old logs to somewhere cheap #329: Dump old logs to S3 instead of losing them
• Set up service monitoring and alert notifications #35: Add Slack as an error logger transport