Migrate all user feedback to GitHub Issues

[coreysreid]

Summary:
Transition all feedback mechanisms (including flashcard feedback during study sessions) from the Django application database to use GitHub Issues exclusively.

Acceptance Criteria:

• Remove the CardFeedback model and feedback-related database logic.
• Remove all in-app feedback forms and admin feedback review dashboards (/admin/feedback/review/).
• Update the "💬 Feedback" button in study sessions to open a pre-filled GitHub issue submission (as the current general feedback button does).
• Ensure feedback context (flashcard/question details) are included in GitHub issue body.
• Remove staff/admin review features that are only for feedback moderation.
• All user feedback should now be tracked via GitHub Issues exclusively.

Rationale:

• Simplifies user and admin experience.
• Leverages GitHub for all issue tracking and auditing.
• Reduces potential GDPR/PPI headaches.

Label: enhancement