refactor: Move get_course_by_id helper from LMS to core

This helper is used by the LMS, CMS, _and_ `openedx.core`,
so let's move it to `openedx.core` to reduce import complexity.

The following files no longer import from LMS:
- cms/djangoapps/contentstore/management/commands/edit_course_tabs.py
- lms/djangoapps/ccx/migrations/0006_set_display_name_as_override.py
- openedx/core/djangoapps/ccxcon/api.py
- openedx/core/djangoapps/verified_track_content/models.py
- openedx/features/course_experience/plugins.py

Note: The LTI XBlock has a dependency on this import path (!?);
a fix can be found here [1].

- [1] https://github.com/edx/xblock-lti-consumer/pull/154

openedx/features/course_experience/plugins.py

@@ -8,8 +8,8 @@ This includes any locally defined CourseTools.
 from django.urls import reverse
 from django.utils.translation import ugettext as _
 
-from lms.djangoapps.courseware.courses import get_course_by_id
 from common.djangoapps.student.models import CourseEnrollment
+from openedx.core.lib.courses import get_course_by_id
 
 from . import DISABLE_UNIFIED_COURSE_TAB_FLAG
 from .course_tools import CourseTool