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]