Resolve "Multilingual push notifications"

Luko van der Maas
2018-03-10T12:14:48+01:00

~feature

Closes #536

### Short description
Added support for multilingual messages notifications
So:
* Added multilingual field to model
* Changed send and admin to work together

### Steps to use this feature

1. Create to different devices with each a different language. (use https://thalia.nu/api/v1/devices/)
2. Create a message with different text for EN and NL and add the user(s) associated with those devices
3. Send the message
4. Watch the notification come in