Add CRUD interface for participation tags

[Copilot]

Participation tags were only manageable via the Django admin interface. This adds first-class course-level views for creating, editing, listing, and deleting participation tags.

Views (course/enrollment.py)

• list_participation_tags — table of all tags for the course
• edit_participation_tag — create (tag_id=-1) or update an existing tag; uses ParticipationTagForm (fields: name, shown_to_participant)
• delete_participation_tag — GET renders confirmation, POST deletes

URLs (relate/urls.py)

• GET/POST /course/<id>/participation-tags/
• GET/POST /course/<id>/edit-participation-tag/<tag_id>/
• GET/POST /course/<id>/delete-participation-tag/<tag_id>/

Templates

• participation-tags.html — tag list with Edit/Delete actions and Add button
• confirm-delete-participation-tag.html — delete confirmation page

Navigation (course-base.html)

"Manage participation tags" added to the Instructor dropdown, gated on pperm.edit_participation (distinct from the existing query_participation gate on "Query participations").

All views require PPerm.edit_participation; cross-course tag access raises SuspiciousOperation → 400.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Add a CRUD interface for participation tags</issue_title>
<issue_description></issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Add a CRUD interface for participation tags #1409

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