Don't set the Cache-Control header on dev sites

Varnish observes the Cache-Control headers on our development server,
which is frustrating when you're actively changing the source and can't
see the changes when you reload the page.

This commit makes sure that TheyWorkForYou only emits Cache-Control
headers when DEVSITE is falsy.
Merge remote-tracking branch 'origin/no-cache-control-for-devsites'