Make banners manageable at scale in the admin (#1082)

[jonfroehlich]

Closes #1082.

Problem

There are ~190 banners on production. The Banner changelist had no filtering, search, ordering, or visual preview, so finding and managing a specific banner meant scrolling an arbitrarily-ordered list and opening rows one at a time. #1082 asked for sort-by-project, sort-by-date, and a right-side filter.

Changes

Overhauled BannerAdmin (website/admin/banner_admin.py):

• Thumbnail column — small easy_thumbnails-generated preview that honors the editor's crop box, with 🎥 video / — placeholders. Keeps the list page light instead of loading ~190 full-size images.
• list_filter — landing_page, favorite, a custom MediaTypeFilter (Has image / Has video / No media), and project. This is the right-sidebar filter requested in the issue.
• search_fields — title, caption, project name, alt text, link.
• date_hierarchy = 'date_added' + default newest-first ordering — the sort-by-date ask.
• list_editable for landing_page and favorite — toggle inline without opening each banner (title pinned as the row link).
• Bulk actions — add/remove from landing page, mark/unmark favorite.

Also removed the dead Banner.admin_thumbnail model method (used the long-removed allow_tags API; unreferenced).

Tests

New website/tests/test_banner_admin.py — covers the thumbnail renderer, the media-type filter logic, and the changelist config (7 tests, all passing). manage.py check is clean.

Notes

• Admin-only change: no model or migration impact, and admin pages aren't Pa11y-scanned.
• A banner with both an image and a video matches both "Has image" and "Has video" (the filters are not mutually exclusive) — documented in the filter docstring.

Screenshots

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