Add `bin/deploy` for Heroku deployments

Heavily influenced by [the blog's
bin/deploy](https://github.com/thoughtbot/robots.thoughtbot.com/blob/92b5c4da00a74979c62002ee4685cbb36cfd0649/bin/deploy),
`bin/deploy {staging,production}` will push to `{staging,production}` and run `rake db:migrate`.

`bin/deploy` will default to staging, but `bin/deploy $REMOTE` accepts an
arbirtrary git remote.
Should we turn maintenance on before migrating and again after migration is complete? If you migrate without maintenance mode, there's a window where requests could reach the application without migrations complete and may result in either application errors or invalid data.