Handle message links (a.message-link)

[gnprice]

The server now emits a new shape of content when the user links to a message:

<a class="message-link"
  href="/#narrow/channel/9-announce/topic/Zulip.20updates/near/214">
 #announce &gt; Zulip updates @ 💬
</a>

This is suddenly pretty common, too, because the web client automatically converts to the syntax for this when the user pastes such a URL.

This is newly merged today. Docs will appear here:
https://zulip.com/api/message-formatting
but for the moment can be read instead here:
zulip/zulip@77064a1

We should handle these. Currently they're showing as "unimplemented".

For this issue, to get it merged quickly, we just want basic support: treat the link like any other link. A fancier feature is to follow renames of the stream/channel; that's a separate issue (#1047) and is out of scope for this one.

• Server/web issue: Format /near/ links nicely zulip#31920
• Server/web PR: markdown: Format /near/ links nicely. zulip#31965
• #api design thread: https://chat.zulip.org/#narrow/channel/378-api-design/topic/notation.20for.20near.20links/near/1972281