Script to deploy staging/production to Heroku

This allows us to deploy to staging or production in a totally consistent
way. The workflow between the two is slightly different:

- Production deployment runs a backup before modifying the database.
- Staging deployment copies in the production database before modifying
 the database.

This ensures that staging is an accurate representation of production for
testing eg database migrations, and that production hopefully doesn't get
destroyed on a borked migration.

https://www.pivotaltracker.com/story/show/73700144

[#73700144]