Add categories for custom emojis (#11196)

Fix #7940

app/models/custom_emoji_category.rb

@@ -0,0 +1,15 @@
+# frozen_string_literal: true
+
+# == Schema Information
+#
+# Table name: custom_emoji_categories
+#
+#  id         :bigint(8)        not null, primary key
+#  name       :string
+#  created_at :datetime         not null
+#  updated_at :datetime         not null
+#
+
+class CustomEmojiCategory < ApplicationRecord
+  has_many :emojis, class_name: 'CustomEmoji', foreign_key: 'category_id', inverse_of: :category
+end