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/lib/xmodule/xmodule/course_module.py

@@ -183,6 +183,15 @@ class CourseFields(object):
                  default=DEFAULT_START_DATE,
                  scope=Scope.settings)
     end = Date(help="Date that this class ends", scope=Scope.settings)
+    cosmetic_display_price = Integer(
+        display_name=_("Cosmetic Course Display Price"),
+        help=_(
+            "The cost displayed to students for enrolling in the course. If a paid course registration price is "
+            "set by an administrator in the database, that price will be displayed instead of this one."
+        ),
+        default=0,
+        scope=Scope.settings,
+    )
     advertised_start = String(
         display_name=_("Course Advertised Start Date"),
         help=_(