Add cleverpush workers

public/cleverpush-manifest.json

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+{"name":"E&P Skireisen","short_name":"E&P Skireisen","start_url":"/","display":"standalone","gcm_sender_id":"890396972010","gcm_user_visible_only":true}

public/cleverpush-worker.js

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+importScripts('https://static.cleverpush.com/channel/worker/MrDvZ9L2bngnLfnq9.js' + self.location.search);