This modifies the logic for formatting outgoing missed-message emails
to support the upcoming stream email notifications feature (providing
a new format for the subject, etc.).
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.
Now, in the event of messages between two other members of a huddle,
the missed message emails are threaded in "Group PMs with name1 and
name2" and not in separate threads by sender.
Also, now the order of recipients in get_display_recipient consistent
with the order of names that appears in the list of personal messages
on the left sidebar.
Fixes most of #4553.
Sarah
Tim Abbott
YJDave
James Rowan
Rishi Gupta