Encode custom emojis as resolveable objects in ActivityPub (#5243)

* Encode custom emojis as resolveable objects in ActivityPub

* Improve code style

app/controllers/emojis_controller.rb

@@ -0,0 +1,22 @@
+# frozen_string_literal: true
+
+class EmojisController < ApplicationController
+  before_action :set_emoji
+
+  def show
+    respond_to do |format|
+      format.json do
+        render json: @emoji,
+               serializer: ActivityPub::EmojiSerializer,
+               adapter: ActivityPub::Adapter,
+               content_type: 'application/activity+json'
+      end
+    end
+  end
+
+  private
+
+  def set_emoji
+    @emoji = CustomEmoji.local.find(params[:id])
+  end
+end