Support multi-question, fully featured problems in the problem editor.

Problem: The new problem editor narrows the instruction design possibilities due to the limitations of the problem editor UI. As a course author, I need to be able to construct complex problems made up of multiple questions and answers in order to evaluate learners' understanding. 

Use Cases: 

A common economics problem type has two steps. First, a graph and a dropdown to identify the type of market, then a numerical entry to identify the point of concern on the graph. From an instructional design perspective, this should be one graded problem since it’s impossible to evaluate the learner’s understanding with just one part or the other. 

Another example would be a prioritization problem, which we achieve with multiple drop-downs. 

Discovery: It was reasonably easy to author these types of problems in the old problem editor with the simple markdown editor, without having to go to the advanced editor. Can the markdown editor be implemented in the MFE and offered conditional on a course waffle flag?