make firebase-admin an optional dependency (#707)

Co-authored-by: Tim Jahn <tim@smartfactory.ch>

docs/FCM.rst

@@ -18,7 +18,7 @@ Initialize the firebase admin in your ``settings.py`` file.
 .. code-block:: python
 
 	# Import the firebase service
-	from firebase_admin import auth
+	import firebase_admin
 
 	# Initialize the default app
 	default_app = firebase_admin.initialize_app()

push_notifications/models.py

@@ -1,9 +1,7 @@
 from django.db import models
 from django.utils.translation import gettext_lazy as _
-from firebase_admin import messaging
 
 from .fields import HexIntegerField
-from .gcm import dict_to_fcm_message
 from .settings import PUSH_NOTIFICATIONS_SETTINGS as SETTINGS
 
 
@@ -60,6 +58,9 @@ def get_queryset(self):
 class GCMDeviceQuerySet(models.query.QuerySet):
 	def send_message(self, message, **kwargs):
 		if self.exists():
+			from firebase_admin import messaging
+
+			from .gcm import dict_to_fcm_message
 			from .gcm import send_message as fcm_send_message
 
 			if not isinstance(message, messaging.Message):
@@ -108,6 +109,9 @@ class Meta:
 		verbose_name = _("FCM device")
 
 	def send_message(self, message, **kwargs):
+		from firebase_admin import messaging
+
+		from .gcm import dict_to_fcm_message
 		from .gcm import send_message as fcm_send_message
 
 		# GCM is not supported.