Split/paver LMS-11211

This PR updates the Paver code for Lettuce Acceptance and Bokchoy tests so we can test them with different modulestores (namely, Split and Draft).

- *Acceptance Tests:*  By default, they will continue to be tested with the Draft Modulestore.  To test with Split, you would need to call paver with --default_store='split'.  Unfortunately, many tests are currently failing with Split and would need to be diagnosed and fixed.  We are not undertaking that work at this time.

- *BokChoy Tests:* By default, Bokchoy tests will now be run with the Split Modulestore.  As part of this story, we had taken the time to fix all the tests that failed with Split.

@jzoldak @cpennington  Have reviewed the commits that changed Paver.
@dmitchell Please review the remaining commits.
@cahrens Please review the last commit related to Studio's drag and drop feature.
It seems like it would be nice to be able to encode this logic in one place. Currently, it's a bit scattered.