Use Firebase SDK batch sending

One-sentence description

Why?

Because it wasn't possible before: https://firebase.google.com/support/release-notes/admin/python#version_2170_-_may_23_2019

Current implementation

No batch sending: https://gitlab.science.ru.nl/thalia/concrexit/blob/master/website/pushnotifications/models.py#L172

Suggested implementation

Batch sending using the new API.