Use recommended setup for Celery

Note: does it improve anything?

gargantext/__init__.py

+from .celery import app as celery_app

gargantext/celery.py

+"""
+Setup the Celery instance (see also gargantext/__init__.py) that will be
+used by all shared_task.
+
+This is the recommended way:
+http://docs.celeryproject.org/en/3.1/django/first-steps-with-django.html
+"""
+
+import os
+from celery import Celery
+
+os.environ.setdefault('DJANGO_SETTINGS_MODULE', 'gargantext.settings')
+
+from django.conf import settings #noqa
+
+app = Celery('gargantext')
+app.config_from_object('django.conf:settings')
+app.autodiscover_tasks(lambda: settings.INSTALLED_APPS)