Add support for new relic.

This reuses requirements/prod.txt to make it actually useful.

For newrelic to work, playdoh.wsgi must be called with the
NEW_RELIC_CONFIG_FILE environment variable set, and optionally the
NEW_RELIC_ENVIRONMENT environment variable set.

Additionally, the browser_monitoring.auto_instrument newrelic setting
must be set to false, or else newrelic will insert JS instrumentation code
into Thimble project source code, confusing users. playdoh.wsgi checks for
this and throws an exception if it's not the case.