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>

db/migrate/20200116092259_add_group_to_custom_emojis.rb

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+class AddGroupToCustomEmojis < ActiveRecord::Migration[6.0]
+  def change
+    add_column :custom_emojis, :group, :string, null: true, limit: 20
+  end
+end