Move zulip-emoji from its current location `images/emoji` to
`images/emoji/unicode` and add a symlink in `images/emoji`
to zulip.png in `images/emoji/unicode`.
Previously, if you searched for ':offi..' you would see both 🏢 and
:office_building: as possible completions, both of which are shortcodes for
the same unicode codepoint (and hence which have the same image). Also, we
sort the emoji in our emoji pickers alphabetically by shortcode, and so the
images for 🏢 and :office_building: show up next to each other, which
looks like a bug. This removes :office_building: as a shortcode, along with
several hundred other duplicates. It leaves some duplicates in that won't
give autocomplete or alphabetical ordering a problem, like (🚗,
:automobile:).
Replaces the hardcoded list of emoji_names and unicode_emoji_names in
static/js/emoji.js with a list generated from emoji_map.json, both to get
the list out of version control and so we can start modifying it for our
autocomplete. This does not change the contents of emoji_names. It sorts and
removes duplicates from unicode_emoji_names (causes no change in behavior,
since unicode_emoji_names is only used as if it were a set).