Undo support for deleting topics

[lukas-shawford]

Deleting a topic deletes all quizzes associated with that topic; it also deletes all subtopics, and all quizzes associated with those topics. As such, it's kind of a dangerous operation, and the user should have the ability to undo it.

There are a couple related issues here:

• The UI should be improved to make it clearer what exactly is going to be deleted when the user deletes a topic. For instance, when deleting a parent topic, the UI should make it clear that all subtopics and their quizzes will also be deleted by calling it out visually (perhaps shade all topics marked for deletion in red and/or animate them in the sidebar; or also list out all the topics and quizzes that will be deleted).
• Perhaps we should not delete subtopics when deleting a parent topic, and instead only delete the parent topic, and move all the subtopics one level up the hierarchy.

TBD:

• Should the ability to undo topic deletion be permanent, or will it only be available immediately after deleting? If it's permanent, then how will that look like - should we have a special section in the sidebar for deleted topics?