Make cache lifetime for static files configurable

[robertlemke]

The cache lifetime sent in an HTTP response as the "expires" header should be configurable via an environment variable.

[robertlemke]

The cache lifetime I set (30d) is too short – Google Lighthouse wants at least half a year.

And we will need changes in the Google Cloud Storage plugin in order to achieve proper cache lifetimes for persistent resources (see flownative/flow-google-cloudstorage#32).