Don't populate DB from rake task.

It's faster to use a dump of the DB via pg_dump and pgbackups:restore
https://devcenter.heroku.com/articles/heroku-postgres-import-export
Update staging.rb to match production.rb
Use db:structure:load instead of db:schema:load

schema.rb doesn't get used or updated since we are using the :sql format
Update puma config per Heroku tutorial

https://devcenter.heroku.com/articles/deploying-rails-applications-with-the-puma-web-server
Switch to unicorn

I was getting lots of R14 and high-response time errors with puma. Trying unicorn to see if it makes a difference.
Populate the DB using PG .dump file

Instead of populating the DB by creating Ruby objects from scratch via the old rake task, we can just use a PG dump file, which is much faster.