[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?

[egeekial]

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"];

My intent was to keep this limited to basic formatting and make the feature something users can simply turn on or off. I could imagine a user wanting finer control, such as allowing italics but not bold or underline, but I suspect most users would not want or need that level of granularity.

If you think that configurability would be beneficial, I would prefer exposing it as a subset of the existing safe list rather than allowing users to add arbitrary tags. Letting users add new tags seems more likely to create rendering issues or introduce an injection risk.

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

Yes, I can do that.