Add prefetch buttons to embedded files

Right now embedded files are automatically downloaded if the files are small (<2MB in 3G, <20MB in WiFi) and when the user opens/plays them with WiFi connection. This means that the user might be downloading a lot of data without knowing it, and there's no way to track the progress of the download.

It would be nice to have a prefetch button for "big" files, like we do with some resources/activities. Also, I think the user should be able to delete already downloaded files (maybe even small files, and mark them somewhere so they aren't redownloaded again).