Support for multi-part quizzes

[lukas-shawford]

For long quizzes than span multiple related concepts, it may be helpful to be able to split up the quiz into multiple "parts". This would provide a logical grouping for the questions that would help orient the quiz taker. It should also hopefully provide for a better UI experience when taking or editing a long quiz (instead of scrolling through a hundred questions, each individual part could be on its own separate page/panel).

When taking a quiz that has multiple parts, the sidebar should display a hierarchical view of the questions within each part (instead of just a flat list). As the user is advancing through the quiz, the "part" should have its own separate page to make it obvious that they have just advanced to a different part.

When editing a multipart quiz, It should be possible to specify a name for each part, as well as any additional info that should appear on the part's intro page (e.g., an explanation of what this part is about, or any special instructions). This should support rich formatting with images.

This is an "advanced" feature - it's important not to sacrifice the usability/ease of use of simple quizzes that don't have multiple parts. As such, the current notion of "flat" quizzes should remain the default.

(TBD: Whether we should allow nesting parts within parts.)