If a Zulip install at example.org got a request at an HTTP `Host`
like foo.example.org.evil.com (or even foo.example.orgevil.com),
we would accept it as subdomain foo. This isn't likely to happen
in practice because it shouldn't pass ALLOWED_HOSTS, and it's not
obvious to me that anything untoward could be done with it even
if ALLOWED_HOSTS were set wide open, but if nothing else it
multiplies the cases in analyzing this logic.
The reason we had a loose match like this, I assume, is to allow
the user to come from arbitrary ports -- especially in development.
So tighten the pattern to allow just that, and add some tests for
that behavior and a comment explaining why this complication is
needed.
Greg Price