Preserve ampersand in profile website URL to fix broken links

What does this MR do and why?

When the feature flag validate_sanitizable_user_details is disabled, profile Website URL was being sanitized with Sanitize.clean(), which encodes & as &. That broke URLs with query parameters (e.g. https://example.com/q=user&somekey became https://example.com/q=user&somekey), so the saved link no longer worked.

This MR keeps sanitization for website_url (to strip HTML/scripts) but restores & after sanitization for that field only, so the stored value remains a valid URL. The same pattern is already used for location and organization. The spec for the legacy path is updated so website_url is expected to preserve &.

References

Resolves : #588853 (closed)

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