Migrate from `realm_filters` event format to new `realm_linkifiers` format

[timabbott]

In zulip/zulip#17596, we changed the format for realm_filters events to a new realm_linkifiers format that is more rational and extensible. We should migrate clients to this new format. (See the backwards-compatibility notes in the API documentation, but I expect a correct implementation will choose whether to send realm_filters or realm_linkifiers in event_types when doing a register post, and then have handled code for GET /events for both formats).

(This will be an important prerequisite for adding stream-specific linkifiers in the future).

[chrisbobbe]

As I understand it, #4242 is needed for this. We do one thing with realm_filters, which is pass it to a parseMarkdown function for local echo. That function was forked from the web app in 2017 and naturally hasn't been kept up-to-date. #4242 is to use shared code, so we don't fall into a situation like that again.

Once #4242 is done, we can take the new event format—presumably the shared code we use will be able to handle it. The next step for #4242 is for Greg to respond to my review of his PR zulip/zulip-markdown-parser#5. Then I can use that PR as a model for further work.

[timabbott]

I'm not sure this needs to block on that -- can't we just write a block of event handler code that takes a realm_linkifiers object and turns it into a realm_filters style tuple before passing the data to the markdown implementation? The format change is I think just obj => [obj.pattern, obj.url_format_string, obj.id]

(And then when moving to a shared markdown implementation, one would potentially switch that around?)

I would expect that as a general rule, we want to handle feature level logic in the code directly adjacent to the network (E.g. a first step after receiving responses from the server) -- so that other code can be clean and simple (either against the old API, or the new, depending on how we want to convert the data).

[chrisbobbe]

The format change is I think just obj => [obj.pattern, obj.url_format_string, obj.id]

Interesting; yeah, if the new realm_linkifiers objects can be converted to old-format realm_filters ones, this shouldn't have to block.

[chrisbobbe]

Just sent #4666.