Support old format for narrow links (subject in addition to topic) #1422
Description
As documented in buttons.py (currently _parse_narrow_link), we support links formatted in the form:
... narrow/stream/[{stream-id}-]{stream-name}/topic/{encoded.20.topic.20name}[/near/{message_id}]
As I noted in #api documentation > Narrow URL formats this doesn't handle an old format where subject was used in place of topic, which makes the URL in the quoted message fail to be identified and link properly in ZT.
Servers running newer versions of Zulip will not generate links of this format, which limits the impact of this issue, but equally anyone who has upgraded an older server will potentially still have messages with links of this form.