Adds a course option to auto-advance videos.

If enabled for a course, as soon as the video ends, the next unit or subsection
will be loaded, and if it contains a single video, that video will be played.

Course authors can enable the setting for a course, but learners can toggle the
setting on or off once it's enabled on the course.

common/test/acceptance/pages/studio/settings_advanced.py

@@ -178,6 +178,7 @@ class AdvancedSettingsPage(CoursePage):
             'course_image',
             'banner_image',
             'video_thumbnail_image',
+            'video_auto_advance',
             'cosmetic_display_price',
             'advertised_start',
             'announcement',