Adapt logging setup to work when the API server is run under gunicorn.

Summary:
Run with:
  gunicorn --worker-class inbox.api.wsgi.InboxWSGIWorker --logger-class inbox.api.wsgi.GunicornLogger inbox.api.srv:app ...

This also requires some minor changes to our existing logging setup, because
gunicorn passes positional arguments in its logging messages
(`self.log.info('message %s', some_value)`) but structlog can't currently
handle that (https://github.com/hynek/structlog/issues/19).

Test Plan:
Run `bin/inbox-api` and `gunicorn --worker-class
inbox.api.wsgi.InboxWSGIWorker --logger-class inbox.api.wsgi.GunicornLogger
--workers=1 --bind 127.0.0.1:5555 inbox.api.srv:app` and make some API
requests.

Reviewers: spang

Reviewed By: spang

Differential Revision: https://review.inboxapp.com/D235