This enables JSON logging for Celery workers if the LOG_JSON conig value
is set. It uses the same JsonFormatter class used by the Flask
applications. That class has been updated in two ways:
- It takes a `source` kwarg to define the log source for the formatter.
- The `msg` attribute of the log record is formatted with any arguments
that may have been passed. This is necessary for Celery to render task
type, completion time, etc. into the log output.
This gives us better test isolation. Previously, we were manually
setting `g.current_user` with a factory instance and not cleaning it up
properly, which could break later tests.
