Add a `/live` monitor for load balancing

[jeremyolliver]

A common deployment pattern is to have multiple application servers behind a load balancer such as nginx or haproxy. HAproxy in particular provides an httpcheck URL, and if that http call doesn't return a successful response, the server will be taken out of rotation.

Addition of a /live route which performs will return an okay response if the individual application server is available AND if it should be included in the live application pool.

Implementation plan:

I've used in existing production setup's a flag file in the application tmp directory, which makes enabling or disabling as easy as touch tmp/LIVE or rm tmp/LIVE