MDL-42666 repository_boxnet: Alert the admins of the migration process
MDL-42666 repository_boxnet: Warn on HTTPS requirement when non HTTPS
MDL-42666 repository_boxnet: Convert all the references asap

This has been done in case the script fails during the processing
and needs to be re-run. Now all the references are converted as soon
as we can, where as before they could have ended up being re-downloaded.
MDL-42666 repository_boxnet: Fix undefined variable usage
MDL-42666 repository_boxnet: Downloading a file follows locations
MDL-42666 repository_boxnet: Improved output during migration