Modified tasks.py to use celery only if available

Please see the comments inside tasks.py.
TL;DR: Defines a fake "task" decorator - the fake
decorator was needed to add the delay() attribute
to the send_notification function

wagtail/wagtailadmin/tasks.py

@@ -1,5 +1,3 @@
-from celery.decorators import task
-
 from django.template.loader import render_to_string
 from django.core.mail import send_mail
 from django.conf import settings
@@ -8,6 +6,27 @@ from django.db.models import Q
 
 from wagtail.wagtailcore.models import PageRevision, GroupPagePermission
 
+# The following will check to see if we can import task from celery - 
+# if not then we definitely haven't installed it
+try:
+    from celery.decorators import task
+    NO_CELERY = False
+except:
+    NO_CELERY = True
+
+   
+# However, we could have installed celery for other projects. So we will also
+# check if we have defined the BROKER_URL setting. If not then definitely we
+# haven't configured it. 
+if NO_CELERY or not hasattr(settings, 'BROKER_URL'):
+    # So if we enter here we will define a different "task" decorator that
+    # just returns the original function and sets its delay attribute to
+    # point to the original function: This way, the send_notification
+    # function will be actually called instead of the the 
+    # send_notification.delay() 
+    def task(f):
+        f.delay=f
+        return f
 
 def users_with_page_permission(page, permission_type, include_superusers=True):
     # Get user model
@@ -25,11 +44,11 @@ def users_with_page_permission(page, permission_type, include_superusers=True):
     return User.objects.filter(q).distinct()
 
 
-@task()
+@task
 def send_notification(page_revision_id, notification, excluded_user_id):
     # Get revision
     revision = PageRevision.objects.get(id=page_revision_id)
-
+    
     # Get list of recipients
     if notification == 'submitted':
         # Get list of publishers