feat: Add course-wide custom scripts

Imlements OEP-15 by adding two fields to the course settings:
- Course-wide Custom JS
- Course-wide Custom CSS
The resources defined in these fields will be rendered in all course pages.

Rebase b6cb629849..0578e1c4c6 onto b6cb629849:
- Add course-wide resources to API for MFE use
- Revert "Add course-wide resources to API for MFE use" reverts commit 53648dcf0afe3cd171c9dc2eb5e56b871b2bcfb2

Signed-off-by: Gabor Boros <gabor.brs@gmail.com>

common/lib/xmodule/xmodule/course_module.py

@@ -998,6 +998,19 @@ class CourseFields:  # lint-amnesty, pylint: disable=missing-class-docstring
         ),
         scope=Scope.settings, default=False
     )
+
+    course_wide_js = List(
+        display_name=_("Course-wide Custom JS"),
+        help=_('Enter Javascript resource URLs you want to be loaded globally throughout the course pages.'),
+        scope=Scope.settings,
+    )
+
+    course_wide_css = List(
+        display_name=_("Course-wide Custom CSS"),
+        help=_('Enter CSS resource URLs you want to be loaded globally throughout the course pages.'),
+        scope=Scope.settings,
+    )
+
     other_course_settings = Dict(
         display_name=_("Other Course Settings"),
         help=_(