Add heroku app.json for one-click deployment

[Alir3z4]

This will be very helpful for others to try it quick.

Requirements

Following are requirements for making the one-click deployment app.

Buildpacks

• heroku/python
• https://github.com/heroku/heroku-buildpack-pgbouncer: PostgreSQL connection pooling.

Addons

• Papertrail:chocklad
• Heroku PostgreSQL - Hobby Dev
• Redis Cloud [30 MB]

Environment Variables

• AUTO_MIGRATE: True
• DJANGO_SECRET_KEY: tosomerandomkey
• PGBOUNCER_MAX_CLIENT_CONN: 19

Note: we set PGBOUNCER_MAX_CLIENT_CONN to 19 because Heroku free PostgreSQL database instance is limited to 20 concurrent connections.