Handle Zulip's encoding of Unicode emoji in messages

[gnprice]

In rendering messages, we should handle this feature: https://zulip.com/help/emoji-and-emoticons#add-emoji-to-a-message

We already (since #4) handle the case where the emoji is an image emoji (i.e., either a realm custom emoji or the Zulip extra emoji, which is :zulip:). The remaining case is where the emoji is a Unicode emoji.

Rough steps to implement this:

• Add more detail to UnicodeEmojiNode, so that it records the sequence of Unicode code points expressed in the CSS class names.
• When building a widget for a UnicodeEmojiNode, show that emoji. Options include:
  • Show that sequence of code points as literal Unicode text, and rely on system fonts to render it. This is what the RN app currently does.
  • Show that sequence of code points as literal Unicode text, and ship a font (like Noto Color Emoji) to render it.
  • Ship the glyphs for the different emojisets, as images. This is what the web app effectively does. (It's actually one image file used as a spritesheet for all the emoji, because that's the way to do it efficiently on web. It's possible that would be the efficient way to do it in Flutter too.)