Recently, one of our users reported that a JIRA webhook was not
able to send messages to a stream with a space character in its
name. Turns out that JIRA does something weird with webhook URLs,
such that escaped space characters (%20) are escaped again, so
that when the request gets to Zulip, the double escaped %20 is
evaluated as the literal characters `%20`, and not as a space.
We fix this by unescaping the stream name on our end before
sending the message forward!
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