New Download Feature to support PWAs

[SebiWrn]

The project already has a service-worker.js to support installing it as PWA. Currently the PWA still requires an internet connection, because no content is stored locally.

New feature is to store Videos encrypted on the users device, so the user can download lectures and watch them offline.
These download should be stored securely and encrypted on the users device with an expiration date.

With that new download feature you can substitute the old one but turn the default mode for new lectures to true.

Maybe also an explanation for lecturers would be helpful.