Enable "jsx-a11y/alt-text" and "jsx-a11y/label-has-associated-control" lint rules

[kerryarchibald]

Enable more jsx-a11y rules

• "jsx-a11y/alt-text"
• "jsx-a11y/label-has-associated-control"

UrlPreviewSettings [no visual change]
Removed unnecessary <label> wrappers (SettingsFlag adds a label internally)
Before:

After:

Checklist

• Tests written for new code (and old code if feasible)
• Linter and other CI checks pass
• Sign-off given on the changes (see CONTRIBUTING.md)

---

This change is marked as an internal change (Task), so will not be included in the changelog.