We want HTML parts to be downloadable, but not displayed as
unreadable (but injection-safe) HTML source in our own web
and Atom interfaces.
This affects indexing, too, as HTML tags/comments won't be
indexed anymore, but existing indices are only cleaned after
--reindex. HTML-only mail won't be indexed at all, but we won't
cross that bridge until somebody cares about that crap. We'll
continue to actively discourage such waste of CPU cycles,
bandwidth, cache and storage.
Fixes: 7d82a8bc04ce2e68 (handle "multipart/mixed" messages which are not multipart')