Improve web push

Extends @egor's !514 (closed) and fixes #393 (closed)

• Documents web push configuration

• Fixes token. It needs token_id (not the actual token string) in order to get a right subscription. This is because a user may have multiple subscriptions on different devices/browsers.

• Improves notification wording.

with:

• Properly disables push if no vapid keys are present (fixes #422 (closed) #439 (closed) #453 (closed))

• Force conversion of subscriptions keys to urlsafe base64

• Fix missing field (notification_type) (Fixes #452 (closed))

• Respect subscriptions alerts settings