Default "Course About" image

Templates that display the image now check if courses specifies an image, if they don't the default image is displayed

Path set in both common.py and aws.py to allow for easy overriding in one place.

Addresses SOL-926

Some code provided by Davorin Sego

lms/djangoapps/courseware/tests/test_module_render.py

@@ -794,16 +794,19 @@ class TestHtmlModifiers(ModuleStoreTestCase):
         self.assertTrue(url.startswith('/static/toy_course_dir/'))
         self.course.static_asset_path = ""
 
+    @override_settings(DEFAULT_COURSE_ABOUT_IMAGE_URL='test.png')
+    @override_settings(STATIC_URL='static/')
     @ddt.data(ModuleStoreEnum.Type.mongo, ModuleStoreEnum.Type.split)
     def test_course_image_for_split_course(self, store):
         """
-        for split courses if course_image is empty then course_image_url will be blank
+        for split courses if course_image is empty then course_image_url will be
+        the default image url defined in settings
         """
         self.course = CourseFactory.create(default_store=store)
         self.course.course_image = ''
 
         url = course_image_url(self.course)
-        self.assertEqual('', url)
+        self.assertEqual('static/test.png', url)
 
     def test_get_course_info_section(self):
         self.course.static_asset_path = "toy_course_dir"