Use the same emoji data on the frontend and backend (#4284)

* Use the same emoji data on the frontend and backend

* Move emoji.json to repository, add tests

This way you don't need to install node dependencies if you only
want to run Ruby code

app/lib/emoji.rb

@@ -0,0 +1,40 @@
+# frozen_string_literal: true
+
+require 'singleton'
+
+class Emoji
+  include Singleton
+
+  def initialize
+    data = Oj.load(File.open(File.join(Rails.root, 'lib', 'assets', 'emoji.json')))
+
+    @map = {}
+
+    data.each do |_, emoji|
+      keys    = [emoji['shortname']] + emoji['aliases']
+      unicode = codepoint_to_unicode(emoji['unicode'])
+
+      keys.each do |key|
+        @map[key] = unicode
+      end
+    end
+  end
+
+  def unicode(shortcode)
+    @map[shortcode]
+  end
+
+  def names
+    @map.keys
+  end
+
+  private
+
+  def codepoint_to_unicode(codepoint)
+    if codepoint.include?('-')
+      codepoint.split('-').map(&:hex).pack('U')
+    else
+      [codepoint.hex].pack('U')
+    end
+  end
+end