Remove links from returned file docs when user is not subscriber/demo/admin.

For unauthenticated/non-subscriber users, image file documents retain
their variations. All other documents have ther variations stripped.
Also the links + expiry info to the original file are removed for all
file types.

tests/common_test_class.py

@@ -79,6 +79,8 @@ class AbstractPillarTest(TestMinimal):
             file = copy.deepcopy(EXAMPLE_FILE)
             if file_overrides is not None:
                 file.update(file_overrides)
+            if '_id' in file and file['_id'] is None:
+                del file['_id']
 
             result = files_collection.insert_one(file)
             file_id = result.inserted_id