SD-373 : Archive ingest task result - back-end implementation

GET /archive_ingest/<task_id>
{'task_id': task_id, 'status': <PROGRESS/PENDING>, 'current': current,
'total': total}
SD-373 : Archive ingest task result - back-end implementation

Changed the celery task to use app as the current flask app.
SD-373 : Archive ingest task result - back-end implementation

Renamed a few functions
fix(archive): fix deleting of items starting with '-'

SD-360
fix(python): make superdesk importable in python2.7

while superdesk might not run there, some parts can be reused