Allow xblocks to be added as advanced problem types

This change allows Studio to add xblocks as advanced problems instead of advanced components, if desired. This feature is being first used to support Peer Assessment as a built-in advanced problem.
That's what I added in lines 997-1005 just above this, or are you thinking of something else. The test adds openassessment into advanced modules and verifies that it is not listed as an advanced component.