Fix assignment update function to not mutate an immutable property

client/src/components/content-blocks/assignment/Assignment.vue

@@ -144,11 +144,12 @@
                 const variables = {
                   id: updatedAssignment.id,
                 };
-                const data = store.readQuery({query, variables});
-
-                data.assignment = Object.assign({}, updatedAssignment, {
+                const assignment = Object.assign({}, updatedAssignment, {
                   submission,
                 });
+                const data = {
+                  assignment
+                };
                 store.writeQuery({query, variables, data});
               }
             } catch (e) {