Autocomplete for emoji in message compose

[gnprice]

This is the third top-level kind of autocomplete in Zulip messages, after @-mentions #49 and #-mentions #124. It triggers on a colon :.

Help Center doc is here (see the "Via Markdown" tab under "Use an emoji in your message"):
https://zulip.com/help/emoji-and-emoticons

The strings this autocomplete acts on are Zulip "emoji names". Zulip has a particular data model for identifying emoji, which is an underdocumented part of the Zulip API. As a prerequisite for this issue, we'll implement that model:

• Track all valid Zulip emoji #669