Since CSS can be overridden by a static webserver on a per-inbox
basis, we need a similar pattern to deal with the instance-wide
WwwListing HTML. "/+/" probably won't conflict with any current
nor future public inbox names.
I don't think it'll cause problems with common linkifiers or URL
extractors, either (and it's unlikely anybody would want to
share URLs of just the CSS in a plain text(-like) format).