Feature Request: Add "Save Draft" button for homework submissions

[dimzachar]

Problem

When filling out a homework submission form, if the user navigates away or page becomes inactive and so on, all entered answers are lost. Users must start over from scratch.

Proposed Solution

Add a "Save Draft" button alongside the existing "Submit" button that allows users to save their progress without finalizing the submission.

Implementation Options

Option A: Minimal (different message only)

• Add "Save Draft" button to the form
• Both buttons save to database identically
• Only the success message differs
• Pros: No migration, minimal code changes
• Cons: Course page shows "Submitted" for drafts (since a submission record exists)

Option B: Full (with draft status tracking)

• Add is_draft boolean field to Submission model
• Add "Save Draft" button to the form
• Drafts show "Open" (or "Draft") on course page instead of "Submitted"
• Pros: Clear distinction between draft and submitted
• Cons: Requires migration

Files affected

• courses/models/homework.py - Add is_draft field (Option B only)
• courses/views/homework.py - Handle is_draft parameter
• courses/templates/homework/homework.html - Add Save Draft button
• courses/views/course.py - Check is_draft when setting status (Option B only)

UI Mockup

[Save Draft]  [Submit]

• Save Draft: Secondary button, saves answers with draft message
• Submit: Primary button, finalizes submission (existing behavior)

Testing

Run existing tests to ensure no regressions:

uv run python manage.py test courses.tests.test_homework
uv run python manage.py test courses.tests.test_course

Manual testing checklist

1. Save draft on new homework → answers persist after page refresh
2. Save draft → course page shows correct status (Open/Draft vs Submitted depending on option)
3. Submit after saving draft → status changes to "Submitted"
4. Scoring still works correctly for submitted homeworks
5. Admin views show correct submission counts

[alexeygrigorev]

If it's a draft but the user forgot to properly submit it, should it be processed when the form is closed?

Also should draft submission allow empty homework url field?

[alexeygrigorev]

Also. Perhaps it could be solved with just cosmetic UI changes? Like adding a note saying that the form allows incomplete submission (plus more details)

[alexeygrigorev]

Implemented in 7cffdef.

We chose the lightweight UX clarification path instead of adding a new draft lifecycle.

What changed:

• Homework submission button now says Save submission for first saves and Update submission for later saves.
• Homework submission status now says Not saved yet or Last saved at, instead of implying a separate final submission state.
• Homework page now explains that students can save partial answers and keep updating until the deadline; the latest saved version is what will be scored.
• Project submission page now uses the same Save submission / Update submission language.
• Project page now explains that students can save the project, keep working, and update the commit ID before the deadline when they have a newer version.

What did not change:

• No new draft model/state.
• No migration.
• No scoring changes.
• Existing save/update behavior remains the source of truth.

Rationale:
The existing flow already supports repeated saves before the deadline. The main confusion was wording: Submit sounded final, even though users can update later. This keeps the behavior simple and avoids introducing draft records that would need separate scoring, status, and close-time semantics.

Verification:

• uv run python manage.py test courses.tests.test_homework courses.tests.test_project_view

Closes #140