Add cache to custom emojis API (mastodon#9732)

Fix mastodon#9729

Author: Gargron

app/controllers/api/v1/custom_emojis_controller.rb

@@ -4,6 +4,8 @@ class Api::V1::CustomEmojisController < Api::BaseController
   respond_to :json
 
   def index
-    render json: CustomEmoji.local.where(disabled: false), each_serializer: REST::CustomEmojiSerializer
+    render_cached_json('api:v1:custom_emojis', expires_in: 1.minute) do
+      ActiveModelSerializers::SerializableResource.new(CustomEmoji.local.where(disabled: false), each_serializer: REST::CustomEmojiSerializer)
+    end
   end
 end