Update Twemoji to 12.1.5 (#13021)

* Update Twemoji to 12.1.5

SVGs from Twemoji repo https://github.com/twitter/twemoji

* Update emoji_map.json to Unicode 12

lib/tasks/emojis.rake

@@ -15,7 +15,7 @@ end
 namespace :emojis do
   desc 'Generate a unicode to filename mapping'
   task :generate do
-    source = 'http://www.unicode.org/Public/emoji/11.0/emoji-test.txt'
+    source = 'http://www.unicode.org/Public/emoji/12.0/emoji-test.txt'
     codes  = []
     dest   = Rails.root.join('app', 'javascript', 'mastodon', 'features', 'emoji', 'emoji_map.json')