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

app/controllers/api/v1/custom_emojis_controller.rb

@@ -0,0 +1,9 @@
+# frozen_string_literal: true
+
+class Api::V1::CustomEmojisController < Api::BaseController
+  respond_to :json
+
+  def index
+    render json: CustomEmoji.local, each_serializer: REST::CustomEmojiSerializer
+  end
+end