Make asset busting depend on deployment environment

At the moment CSS and JS resources have a cache busting tag at the end of their href/src to force a reload when re-rendered. This is deependant on an unused setting, and is present in prod as well, this is really only a feature for development. So, it's been a PITA when diffing outputs

Testing Done:
I've run the `hugo` and `hugo serve` commands with deployment env between prod and dev, and behaviour is as expected

Reviewed at https://reviews.imfreedom.org/r/145/