add exam authors to collection/exam base

generic/mixins.py

@@ -29,3 +29,11 @@ class AuthorMixin():
         if not authors:
             return "None"
         return authors
+
+    def add_author(self, user: User):
+        """Add an author to the object"""
+        self.author.add(user)
+
+    def remove_author(self, user: User):
+        """Remove an author from the object"""
+        self.author.remove(user)