Allow channel metadata and content node endpoints to be filtered by channel based quizzes

[rtibbles]

In order to enable the workflow in #7862, it will be necessary to filter channels based on whether they contain channel based quizzes, and also to filter topics and exercises by whether they have channel based quizzes.

A few possible approaches:

• Filter on the options field with a regular text/re search for "modality": "QUIZ" - would require search the entire descendant tree for topics and channels
• Add a model field to annotate whether something is or contains a channel based quiz for quicker lookup
• Use whoosh or some other search indexing to allow flexible search to add different arbitrary fields