[newsfeed] add allowBasicHtmlTags option for basic emphasis

[egeekial]

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Render a strict allowlist of basic formatting tags (b, strong, i, em, u) in news titles and descriptions, while neutralizing all other HTML.

Feeds such as The Atlantic encode emphasis as entities (<em>), which html-to-text decoded to a literal string that the template then auto-escaped, so the raw tag was shown on screen. The new opt-in allowBasicHtmlTags option (default false) sanitizes both fields by escaping everything and restoring only the exact, attribute-free allowlisted tags, so the result is safe to render and arbitrary HTML/script injection is impossible.

Adds unit tests for the sanitizer and an e2e test covering rendering and an injection attempt.

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[sdetweil]

do you ever envision the user wanting/needing to adjust the list of tags

in newfeedfetcher.js

const ALLOWED_TAGS = ["b", "strong", "i", "em", "u"];

if this is approved, will you also create a matching PR for the documentation repo?