Add categories for custom emojis (#11196)

Fix #7940

db/migrate/20190627222225_create_custom_emoji_categories.rb

@@ -0,0 +1,9 @@
+class CreateCustomEmojiCategories < ActiveRecord::Migration[5.2]
+  def change
+    create_table :custom_emoji_categories do |t|
+      t.string :name, index: { unique: true }
+
+      t.timestamps
+    end
+  end
+end

db/migrate/20190627222826_add_category_id_to_custom_emojis.rb

@@ -0,0 +1,5 @@
+class AddCategoryIdToCustomEmojis < ActiveRecord::Migration[5.2]
+  def change
+    add_column :custom_emojis, :category_id, :bigint
+  end
+end