ENH: Expose issue sections in issue page context

[kopfduenger]

Summary

This adds an issue_sections context variable to the journal issue view.

issue_sections is built from the issue’s unpaginated, sorted, published articles and preserves the section order while removing duplicate sections.

Rationale

Issue templates may need a section-level table of contents or navigation. The existing articles context is paginated, so grouping articles by section can miss sections that only appear on later pages.

Providing issue_sections gives themes a stable section list independent of article pagination, while keeping the existing paginated articles context unchanged.

Implementation

• Reuses issue_object.get_sorted_articles() once in the issue view.
• Builds issue_sections before pagination.
• Keeps articles paginated as before.
• Exposes issue_sections alongside the existing issue context.

Tests

Added a regression test that verifies:

• issue_sections includes a section whose article falls beyond the first paginated page.
• issue_sections excludes sections that only contain unpublished articles.
• The existing paginated articles context remains paginated.

Test run:

lando python manage.py test journal.tests.test_journal_frontend.TestJournalSite.test_issue_sections_context_is_not_paginated