Update implementation for subject_links -> topic_links transition

In https://github.com/zulip/zulip/pull/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.