Simplify repository API to sync files

Enable and set up repositories box.net, dropbox, filesystem and equella, add files in each of repositories Create a course with shortcuts to files in all repositories above, a shortcut to file in teacher's private files, and a shortcut to a file in another module of the same course (accessed via Server files). Usually it's easier to create file or folder resource. Make sure that you can see the correct file size for all those references. To see the filesize you need to edit the resource and click on the file in filemanager. Note that there is a different function responsible for downloading a file and it was not changed in this issue. Overwrite the files in all repositories with another one that has different size. Make sure that you can see the correct file size for all references. Note that local repositories should update immediately, filesystem has 1 minute cache lifetime but box.net, dropbox and equella have 24h cache lifetime. You might want to hack the code of function sync_reference() in those repos. Backup the course Delete the course Restore the course Make sure all filesizes are correct Delete the course Update files in all repositories with files with another size Restore the course Make sure all filesizes are correct Make sure that you can download all files (checking for regressions)