Merge branch '280-is_member_of-predicate' into 'master'

Resolve "is_member_of predicate"

Closes #280

See merge request AlekSIS!290

aleksis/core/util/predicates.py

@@ -98,6 +98,12 @@ def is_group_owner(user: User, group: Group) -> bool:
     return group.owners.filter(owners=user.person).exists()
 
 
+@predicate
+def is_group_member(user: User, group: Group) -> bool:
+    """Predicate which checks if the user is a member of the provided group."""
+    return user.person in group.members.all()
+
+
 @predicate
 def is_notification_recipient(user: User, obj: Model) -> bool:
     """Check if is a notification recipient.