FEATURE: allows multiple custom emoji groups (#9308)

Note: DBHelper would fail with a sql syntax error on columns like "group".

Co-authored-by: Jarek Radosz <jradosz@gmail.com>

app/models/custom_emoji.rb

@@ -14,6 +14,7 @@ end
 #  id         :integer          not null, primary key
 #  name       :string           not null
 #  upload_id  :integer          not null
+#  group      :string
 #  created_at :datetime         not null
 #  updated_at :datetime         not null
 #