Linkifier with automatic Fallback

[xeruf]

I have a Zulip server that is about a GitHub organization, which has a few repositories. Thus, it would be nice if pasting a commit hash from any of the repositories would automatically link to the correct repo.

The solution I thought of is a Linkifier with fallback: You provide a default (i.e. url for repo1) and when that returns a 404 for a particular input, there is an alternative (url for repo2). This could be chained and Zulip would simply go through each link of the chain and find the first one that doesn't return a 404, and if all are 404, then don't turn it into a link at all.
The extra (milli)seconds this adds until the link is inserted wouldn't really bother since it only has to be performed once when the message is sent, but the extra convenience would be amazing and this could be used for far more than just commit hashes.

Discussion: https://chat.zulip.org/#narrow/stream/2-general/topic/Linkifier.20with.20automatic.20fallback

[zulipbot]

Hello @zulip/server-markdown, @zulip/server-settings members, this issue was labeled with the "area: markdown", "area: settings (admin/org)" labels, so you may want to check it out!

[timabbott]

This would require a bit more architecture work than it might seem, since currently we don't fetch the link at all for realm filters in the markdown rendering phase. But would still be a nice improvement.

I'm tagging this as difficult since the refactoring involved will require some careful design work.