Support individual_emoji_changes client capability

The server is adding a new client capability individual_emoji_changes. We should add support for the new API that it controls, and start sending the capability.

This will involve supporting two new event types RealmEmojiAddEvent and RealmEmojiUpdateOneEvent.

This is useful because the old API is subject to accidental DDoS when a realm adds a large number of custom emoji. Moreover, once we have a mobile release out that supports the new API, the clock starts on being able to later remove the old API from the server.

References

• #api design > realm_emoji update is quadratic
• https://github.com/zulip/zulip/pull/38787