Bug: the html content of emails is ignored if html2text is not available

If the GPL library html2text is not available, then any html parts of mails are ignored for the summary display.

Such parts should either be treated as an attachment, or shown as the body text or perhaps both, as is done by mod_mbox.

No file, test, or entry point is named. Start by tracing the email summary handling and the fallback used when html2text is unavailable; before coding, clarify whether HTML should be shown as body text, treated as an attachment, or both, then define the corresponding regression coverage.

Written by the indexing model from the issue text.