refs T8924

This is a proposed solution for Web Push notifications for Phabricator.
Supported apps: Conpherence

Later improvements:

• display details in notifications instead of the generic information
• clicking on the notification should open conpherence chat page
• only send push notifications if a user has been inactive for some time
• wait for Firefox support not to be that flaky

• run phabricator on a secure url
• create a project in https://console.developers.google.com/project
• enable GCM and set project ID and API key to webpush.gcm-sender-id and webpush.gcm-api-key respectively