In the rare case that Zulip receives an email with only an HTML
format, we originally (code dating to 2013) shelled out to
html2markdown/python-html2text in order to convert the HTML into
markdown.
We long since added html2text as a reasonably managed Python
dependency of Zulip; we should just use it here.
Note that Freshdesk allows custom templating for outgoing payloads
in their webhook UI. Therefore, the payloads added in this commit
did not have to be official payloads from Freshdesk.