[smtp] add option to send html emails

[signebedi]

When we send custom jinja2 emails, it may sometimes make sense to allow emails. We should of course always default to plaintext. Adding html is, in theory, as simple as running

                msg.attach(MIMEText(content, 'html'))

[signebedi]

This is now the default behavior. There may be some difficulties, so we'll test the email config, which now default to HTML. Existing instances will need to update their email configurations. This issue is related to #284.