Admin improvements


<img width="755" alt="image" src="https://github.com/anfema/firebase_push/assets/8401820/e0951b03-de46-4bc9-ab07-6e69e9c2e7ba">
- add helptexts
- make body a textfield
- make user dropdown a raw id field


FCM history:
- raw id field for device dropdown
- raw id field for user dropdown
- readonly fields, if created_at/updated_at exist