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`)
This implies that it's enabled by default. If you haven't configured Apache or NGINX to serve files, set this to `true` (or set the `SERVE_STATIC_ASSETS=true` env var) to enable serving the files in your `public/` directory. Setting to `nil` (the default), `false`, or `"false"` disables static asset serving.