Update newrelic

Instead of symlinking untracked `config/newrelic.yml` on the server,
which isn't 12 Factor, use `ENV` variable to read in API key and
customize application name.
Update `config/deploy.rb` for new deployment setup

`config/newrelic.yml` and `config/sidekiq.yml` are no longer needed
because `newrelic` is configured with `ENV` variables and
`config/sidekiq.yml` isn't being used anymore.

The `public/closing` directory is no longer relevant.