Rank emoji autocomplete results

[gnprice]

For the initial implementation of #670, there won't be a useful ordering to the results. This means a user might have to type a longer, more specific query to find their desired emoji than they might with good ranking.

This issue is for ranking the results. The ranking should match what the web app does:
https://github.com/zulip/zulip/blob/65f05794e/web/shared/src/typeahead.ts#L322-L382
(We might make exceptions and depart from that ordering if there are areas where we see an alternative that's substantially better in either quality of results or clarity of code, without being particularly worse on the other axis.)

Broadly, that ranking looks like:

• "Popular emoji", before realm emoji, before (other) Unicode emoji, where "popular emoji" means a hard-coded list of six emoji like 👍.
• Names that exactly match the query, before names that have it as a prefix, before other matches.

For details, see the web implementation.

Related issues

• Prerequisite:
  Autocomplete for emoji in message compose #670
• Similar features for other types of autocomplete:
  autocomplete: Sort user-mention autocomplete results #228
  autocomplete: Give preference to subscribed users first in @-mention autocomplete #618
  In topic autocomplete, rank results by match relevance #881

[gnprice]

I decided this is a nearer-term priority after trying out the emoji picker from #1103 — the lack of ranking was bugging me basically every time I used the emoji picker. So I've now sent #1112 to implement it.