Use uuid() to generate unique XQueue submission IDs during import

This is a fix for the `import_graded_essays` Django management command.  Previously, this generated unique XQueue submission IDs using `test_util.generate_new_xqueue_id()`.  Unfortunately, that method randomized the length of the ID between 1 and 10, so we got lots of IDs of the form "aR".  Running the command multiple times would result in ID collisions, causing database errors.

@adampalay