Fix emojify() by generating a mapping to existing Twemoji files (#5080)

A new rake task emojis:generate downloads a full list of valid
unicode sequences from unicode.org and checks it against existing
Twemoji files, finally generating a map from each sequence to the
existing file (e.g. when there's multiple ways an emoji can be
expressed). The map is dumped into app/javascript/mastodon/emoji_map.json

That file is loaded by emojione_light.js (now a misnomer) which
decorates it further with shortcodes taken from emoji-mart's index.

app/javascript/mastodon/emojione_light.js

@@ -1,13 +1,38 @@
 // @preval
-// Force tree shaking on emojione by exposing just a subset of its functionality
+// http://www.unicode.org/Public/emoji/5.0/emoji-test.txt
 
-const emojione = require('emojione');
+const emojis         = require('./emoji_map.json');
+const { emojiIndex } = require('emoji-mart');
+const excluded       = ['®', '©', '™'];
+const skins          = ['🏻', '🏼', '🏽', '🏾', '🏿'];
+const shortcodeMap   = {};
 
-const mappedUnicode = emojione.mapUnicodeToShort();
-const excluded = ['®', '©', '™'];
+Object.keys(emojiIndex.emojis).forEach(key => {
+  shortcodeMap[emojiIndex.emojis[key].native] = emojiIndex.emojis[key].id;
+});
 
-module.exports.unicodeMapping = Object.keys(emojione.jsEscapeMap)
-  .filter(c => !excluded.includes(c))
-  .map(unicodeStr => [unicodeStr, mappedUnicode[emojione.jsEscapeMap[unicodeStr]]])
-  .map(([unicodeStr, shortCode]) => ({ [unicodeStr]: [emojione.emojioneList[shortCode].fname.replace(/^0+/g, ''), shortCode.slice(1, shortCode.length - 1)] }))
-  .reduce((x, y) => Object.assign(x, y), { });
+const stripModifiers = unicode => {
+  skins.forEach(tone => {
+    unicode = unicode.replace(tone, '');
+  });
+
+  return unicode;
+};
+
+Object.keys(emojis).forEach(key => {
+  if (excluded.includes(key)) {
+    delete emojis[key];
+    return;
+  }
+
+  const normalizedKey = stripModifiers(key);
+  let shortcode       = shortcodeMap[normalizedKey];
+
+  if (!shortcode) {
+    shortcode = shortcodeMap[normalizedKey + '\uFE0F'];
+  }
+
+  emojis[key] = [emojis[key], shortcode];
+});
+
+module.exports.unicodeMapping = emojis;