Enable Heroku Review Apps

We currently have 3 pre-production environments which we use to test changes, but to facilitate a larger dev team, we should look into using Heroku review apps.

https://devcenter.heroku.com/articles/github-integration-review-apps

**Checklist**

- [ ] Update app.json

- [ ] Configure review apps in Heroku

- [ ] Set up database for review apps

**Marlon Keating Aug 19 at 1:32 PM**
My in-progress branch for reference: DemocracyLab/CivicTechExchange: Pull Request 359

https://trello.com/c/tU4yNBcM

## Prior Attempts ##
#359