Extend PencilBannerSnippet placement to HomePage, ArticleThemePage, and ArticleDetailPage.

[slightlyoffbeat]

Summary

Extend PencilBannerSnippet placement to HomePage, ArticleThemePage, and ArticleDetailPage.

Changes

• New *PencilBannerPlacement Orderable models for the three page types (Wagtail's ParentalKey can't target multiple page types, so one class per page).
• "Pencil Banners" InlinePanel added to each page's content_panels.
• Render loop hoisted from free_form_page2026.html into base-flare26.html behind an is defined guard, matching the existing pattern used for data-promoted-page. Any page exposing pencil_banner_placements now auto-renders.
• One parametrized test replaces three near-identical rendering tests.
• Migration 0104: three CreateModel ops, purely additive.

Test plan

• python manage.py migrate
• pytest springfield/cms/tests/test_pencil_banner_snippet.py
• In Wagtail admin: attach a published PencilBannerSnippet to HomePage, an ArticleThemePage (e.g. /features/), and an ArticleDetailPage (e.g. /features/protection/). Confirm banner renders at the top of each page and dismiss button works.
• Confirm existing FreeFormPage2026 pages (e.g. World Cup) still render their banner unchanged.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.22%. Comparing base (b653695) to head (110e185).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1506      +/-   ##
==========================================
+ Coverage   80.90%   81.22%   +0.31%     
==========================================
  Files         147      147              
  Lines       10133    10160      +27     
==========================================
+ Hits         8198     8252      +54     
+ Misses       1935     1908      -27     

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