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).