Show emoji-only messages at larger size

[gnprice]

When a message consists only of (a) one paragraph which (b) consists entirely of one or more emoji, we plan to start showing the emoji at a larger size.

This is a purely client-side change, when displaying the message. The web app will be doing this soon, and we should match the same behavior in the mobile app.

References

• #api design > large emojis @ 💬
• #design > emoji-only message presentation
• https://github.com/zulip/zulip/issues/15390
• https://github.com/zulip/zulip/pull/36681
• API docs (as screenshot): #api documentation > new feature level: 436 @ 💬