if we are in a heroku preview environment, don't build database, just use staging db

[fgregg]

the PR previews take a very long to build because we have to build the entire database. let's not do that. let's just use the staging db.

we do something like this for chicago councilmatic, where the release script checks if we are in the production environment and conditionally runs commands. We could and should also check if we are in the staging environment.