Update release command to create readonly user

On release, in addition to running database migrations on the Postgres
database, ensure that there exists a 'readonly' user for the Arango
database, with universal read-only permissions. This user is used by
application code for certain actions like running arbitrary AQL queries
via the GET /api/workspaces/{workspace}/aql/ endpoint.

Procfile

@@ -1,3 +1,3 @@
-release: ./manage.py migrate
+release: ./manage.py migrate && ./manage.py createarangoreadonlyuser
 web: gunicorn --bind 0.0.0.0:$PORT multinet.wsgi
 worker: REMAP_SIGTERM=SIGQUIT celery --app multinet.celery worker --loglevel INFO --without-heartbeat