Handle "general chat" in rendering #-mentions

[gnprice]

When the user types a #-mention of a stream/channel, a topic, or a message, the a element in the resulting HTML has a class to mark it as such. See docs here: https://zulip.com/api/message-formatting

Currently we just ignore that class and treat the element the same as any other link. But coming soon, Zulip web will rewrite the topic name in one case: when a #-mention of a topic or message points to a topic that is the empty string, i.e. the special "general chat" topic, the client will rewrite the topic name with the display name it uses for that topic elsewhere in the UI per #1250. (It already rewrites the channel name to handle renames: #1047.)

So we should match that behavior.

References

• Handle channel renames in rendering #-mentions #1047
• #api design thread

[gnprice]

I've slotted this as M6 Post-launch because it's not a critical part of the "general chat" feature: it only comes up if the user has their UI in a different language from their realm's default.

OTOH it's higher-priority than the related feature #1047 (currently at M7 Future), because it's probably more common than channel renames, provided this upcoming "general chat" feature gets widely used. (Which we're expecting it will; else we wouldn't be spending as much effort on it, across Zulip as a whole, as we are.)