Support Markdown-rendered entries

[simonw]

Summary

• add a use_markdown flag to entries with a rendered helper and migration, updating related admin and validation logic
• replace the entry HTML helper filters with BeautifulSoup parsing to support Markdown output
• render markdown-backed entries across templates and feeds and add regression coverage for all key pages

Testing

• python manage.py test -v3

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https://chatgpt.com/s/cd_68e1727b970c819183e8e3d56914a251