Add push notifications for newsletters

Mario Tsatsev
2020-02-05T22:31:42+01:00

/label ~feature

Closes #922

### Short description
When the newsletter is sent the users will receive notification about it through the app

### Steps to use this feature

1. Create a newsletter as usual
2. Sent the newsletter as usual

### Expected result

Thalians will receive a notification about the newsletter on their phones.