When showing downloaded sizes in the context menu and in the 'Manage
storage' screen, show the total size including downloaded files plus
cached web service data.
When deleting the downloaded data in either location, delete both
kinds of data.
Adds a 'Manage storage' page on the course menu. This page
allows users to see how much file storage is being used by
each activity, section, and the whole course, and delete
the files if required.