Allow static asset serving from env variable

Based on conversation in #16464 we can't enable this by default for ALL applications. It was agreed that it's reasonable if the host container can set this value (i.e. docker/heroku). 

It's important that we can explicitly enable as well as disable this value as services may enable setting environment variables to different values (like `"true"` or `"false"`) but not allow unsetting the values (`nil`)