Track all valid Zulip emoji

[gnprice]

This is a prerequisite for:

• Support adding an arbitrary reaction #388
• Autocomplete for emoji in message compose #670

In order to offer the user a UI to choose an emoji from the emoji Zulip knows about, we'll need to know what emoji those are and what Zulip calls them.

Zulip has a particular data model for identifying emoji, which is an underdocumented part of the Zulip API. For an introduction, see reaction_type here:
https://zulip.com/api/add-reaction#parameter-reaction_type
and the two companion fields emoji_code and emoji_name. (The same emoji model is used for reactions as for emoji in messages.)

Then see zulip-mobile's src/emoji/data.js, which tracks this model.

A good implementation of this issue will:

• Cover all three of Zulip's types of emoji.
• Support efficiently looking up an emoji by name, or by type and code.
• Support iterating through the list of all Zulip emoji.