Make lode be able to optionally serve over HTTPS (behind a reverse proxy)

[LucaPrete]

Lode is currently not able to optionally serve files over HTTPS, if behind a reverse proxy.

This is due to two issues:

1. The servlet hardcodes the http:// prefix while loading css/js resources -> should be changed to // to automatically adapt to the schema
2. The jetty configuration needs to be instructed to forward https headers to the application so that it can automatically adapt to the schema in use