Update implementation for subject_links -> topic_links transition

[timabbott]

In zulip/zulip#13843 merged for Zulip 2.2, we migrated the server from the legacy subject_links field (named after the legacy "subject" field name) to topic_links.

My understanding is the mobile app never used the feature; in the webapp it is used to render the little link boxes that show up if you have a link or a reference to a linkifier in the message's topic (we need a separate widget because clicking on the topic itself navigates to that topic). I'm not sure what the mobile UI would be for this feature, but probably something in a long-press menu or something.

So this is mainly an issue so you can migrate references in the codebase types, and maybe write a shim to support older servers while one is thinking about this.

A correct backwards-compatability approach for servers 2.1 and older would be to copy a subject_links field to topic_links if present.

[chrisbobbe]

Adding "experimental UI/UX". Although support for copying links to messages is in progress at #3865, a feature for copying topic or stream links isn't tracked yet, as far as I know.

[gnprice]

For previous discussion of the specifics of where and how this field appears in the mobile codebase, see:
zulip/zulip#13588 (comment)

[sumanthvrao]

Howdy! It's worth mentioning here that with zulip/zulip#17124, the topic_links format would now contain a list of objects rather than the list of URL strings. This means that the mobile code to handle this would have to be tweaked in order to support all server versions. Also, the new format benefits from having the text along with the url for each link in the topic, thus simplifying any client-side parsing logic.

[chrisbobbe]

Great, thanks for the heads-up! 🙂