E2605: Complete frontend support for specialized rubrics by topic

[PositivelyBookish]

Summary

This PR completes the frontend support for specialized rubrics by topic in the assignment editor.

It adds the UI and request handling needed for instructors to:

• enable Review rubric varies by topic?
• view review-rubric selectors for each topic in the Topics tab
• assign a different review rubric to each topic
• preserve the assignment-level review rubric as the default fallback when no topic-specific override is selected

Changes

• persist vary_by_topic state in the assignment editor
• load topic-specific rubric mappings from the backend when editing an assignment
• render per-topic review rubric dropdowns in the Topics tab when topic-specific rubrics are enabled
• support the combined topic + round case by rendering one rubric selector per topic per round when applicable
• save topic-specific rubric selections back to the backend through the topic-rubric API flow
• align assignment editor request/response handling with the backend rubric-by-topic contract
• include a small questionnaire form fix so locally created questionnaires/rubrics can be submitted correctly during manual testing

Behavior

After this change, the assignment editor supports all expected rubric-selection cases:

• rubric does not vary by round or topic
• rubric varies by round only
• rubric varies by topic only
• rubric varies by both round and topic

When Review rubric varies by topic? is enabled:

• each topic gets its own review-rubric selector
• the selected topic rubric overrides the assignment default
• topics without an override continue using the default assignment review rubric

Manual verification:

• enabled topic-specific rubrics in the assignment editor
• created topics
• selected different rubrics per topic
• reloaded the page and confirmed selections persisted