We now include whether the message was a private or group private
message; this is particularly important with the new setting to
disable including any message content in these emails (since in that
case, one doesn't know anything about the message types).
Make sure 's, &s, and other characters are not HTML-escaped in subject
lines and plain-text emails.
Hack so that this isn't blocking the release of Zulip 1.6. A more robust way
to do this would be to have two different template Engines, one that renders
HTML, and one that doesn't.
Fixes#5088.