Remove unnecessary comments in database.yml
DRY up database.yml

To avoid repetition, I extracted the common settings into its own block
and referenced it through all environments.
Use `secret_token` instead of `secret_key_base`

`secret_key_base` is new in Rails 4, and replaces `secret_token`. We
don't need to be backwards-compatible, so I removed `secret_token`
altogether.