Register a web push subscription

Stores a browser Web Push subscription for big-story notifications. TechSnif only notifies for tier 3 (top) stories, at most one story per pipeline run, at most one push every 3 hours, and at most 3 pushes per UTC day. Send the object produced by PushSubscription.toJSON(); the endpoint must be an https URL. Re-subscribing with the same endpoint refreshes the stored keys.