move callbacks to controllers and hooks where they are required to avoid...

... loading master files on each media object save

It's preferable not to iterate through each MasterFile when saving a MediaObject.  The user would move a section and it would take ~20 seconds for the ajax call to finish on a 14 section MediaObject.  If they click save while that is happening they will see the sections in the same order before the ajax call.

We could add a loading indicator, but I'm not entirely sure how multiple ajax reordering requests will be handled.  It now occurs to me that it might be beneficial to remove the ajax call and send ordered sections to the update action when the user clicks save.   The increased performance from the changes in this pull request might make that option workable.

Reference:
https://gist.github.com/atomical/3282850b67f1e4285667