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

respect hub.lease_seconds (fix #46)

app/controllers/api/subscriptions_controller.rb

@@ -4,6 +4,7 @@ class Api::SubscriptionsController < ApiController
 
   def show
     if @account.subscription(api_subscription_url(@account.id)).valid?(params['hub.topic'], params['hub.verify_token'])
+      @account.update(subscription_expires_at: Time.now + (params['hub.lease_seconds'].to_i).seconds)
       render plain: HTMLEntities.new.encode(params['hub.challenge']), status: 200
     else
       head 404