New API: GET /api/v1/custom_emojis to get a server's custom emojis (#5051)

app/serializers/rest/custom_emoji_serializer.rb

@@ -0,0 +1,11 @@
+# frozen_string_literal: true
+
+class REST::CustomEmojiSerializer < ActiveModel::Serializer
+  include RoutingHelper
+
+  attributes :shortcode, :url
+
+  def url
+    full_asset_url(object.image.url)
+  end
+end