Add author validation in collection_take_start and implement is_author method in AuthorMixin

generic/mixins.py

@@ -63,6 +63,10 @@ class AuthorMixin():
         """Remove an author from the object"""
         self.author.remove(user)
 
+    def is_author(self, user: User) -> bool:
+        """Returns True if the user is an author of the object"""
+        return self.author.filter(id=user.id).exists()
+
 
 class UserConfigurablePaginationMixin:
     default_per_page = 25