formatter: format element robustness fix

* Fixes issue when certain variables where when None would cause a TypeError
  exception.

* Fixes PEP8 and PEP257 issues in file.

Signed-off-by: Lars Holm Nielsen <lars.holm.nielsen@cern.ch>
logging: celery logging to sentry fix

* Adds support for logging to Sentry from Celery tasks.

* Adds support for modifying Sentry transport for Celery only.

* Fixes legacy issue with use of invenio.config.

Signed-off-by: Lars Holm Nielsen <lars.holm.nielsen@cern.ch>