URL: Prefetch display type and description embedded file when prefetching a course

In MOBILE-2539 we added support for appearance options. This requires a WS call that won't be prefetched if a course is downloaded. Also, embedded files in the description won't be downloaded either.

Displaying a download icon in each URL can be confusing because the user could think that the contents of the URL will be downloaded, but that's not the case. I tried to implement a special prefetch handler that doesn't display the icon, but then it caused problems with section & course download.