[client-narratives] modify markdown parser settings
[client-narratives] modify markdown parser settings

Parsing of the narrative (YAML) frontmatter produces markdown with `<sup>` and `<sub>` HTML elements to render affiliations using superscripts. (This behavior is unchanged from parsing server-side.) The client-side markdown sanitizer settings, however, were stricter than those employed on the server, and were removing these HTML elements causing the affiliations not to be displayed. These settings are relaxed in this commit.
nextstrain/auspice,nextstrain/auspice,nextstrain/auspice