Use ugettext_noop instead of lambdas assigned to the underscore

Replaces instances of an old hack with the more clear ugettext_noop.

common/lib/xmodule/xmodule/imageannotation_module.py

@@ -13,7 +13,8 @@ from xblock.fragment import Fragment
 
 import textwrap
 
-# Make '_' a no-op so we can scrape strings
+# Make '_' a no-op so we can scrape strings. Using lambda instead of
+#  `django.utils.translation.ugettext_noop` because Django cannot be imported in this file
 _ = lambda text: text