Cron jobs

Periodic jobs for-
1. Delete trashed items
2. Unused photos
3. Expired seller locations.

Also added signals to auto-delete photo when changing product or vendor photo.

Setup:
Install :   celery, django-celery   (using pip install)

Update database schema:  
python manage.py makemigrations 
python manage.py migrate

To start the periodic jobs:
 python manage.py celeryd -B -l INFO 

The output can be saved in some file using shell redirection.
Once the jobs are started, they will run periodically every midnight.