MDL-33093 invalidate the thumbnail in the local browser's cache on update

MDL-33093 invalidate the thumbnail in the local browser's cache on update

This adds &oid= parameter to the URL of file preview images. The
parameter is supposed to be ignored by draftfile.php and pluginfile.php.
It just holds some unique identification of the original file's
revision. Currently, we use the timestamp of the most recent
modification of the original file.