Separate PuSH subscriptions from following, add mastodon:push:refresh task,

respect hub.lease_seconds (fix #46)

app/services/follow_service.rb

@@ -12,6 +12,7 @@ class FollowService < BaseService
     if target_account.local?
       NotificationMailer.follow(target_account, source_account).deliver_later
     else
+      subscribe_service.(target_account)
       NotificationWorker.perform_async(follow.stream_entry.id, target_account.id)
     end
 
@@ -40,4 +41,8 @@ class FollowService < BaseService
   def follow_remote_account_service
     @follow_remote_account_service ||= FollowRemoteAccountService.new
   end
+
+  def subscribe_service
+    @subscribe_service ||= SubscribeService.new
+  end
 end