Add cosmetic price field to studio & logic

- Price field added to Advanced Settings
- Function that decides which price to display
- Test added to check that the function outputs correct price
- Added feature flag and conditional to decide whether to display course price or not
- Feature Flag to show or hide course price on Course About page sidebar when not using shopping cart

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

@@ -155,6 +155,7 @@ class AdvancedSettingsPage(CoursePage):
             'certificates_display_behavior',
             'cohort_config',
             'course_image',
+            'cosmetic_display_price',
             'advertised_start',
             'announcement',
             'display_name',