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.
To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information