fix: roles cache organization case-insensitive

[shadinaif]

Description

RoleCache compares org in a case-sensitive manner while the value of org is assumed as case-insensitive by the database collation, and by the django admin form. This PR is to fix the issue to make RoleCache behave as the rest of the platform

Supporting information

Testing instructions

Preperation to test:

• Assume a normal user in the platform, for example test_user
• Assuming we have two courses in demo organization: course-v1:Demo+topic1+index1 and course-v1:demo+topic2+index2. We have the first course with organization Demo in course-overview instead of demo. This is acceptable in the platform

Before the fix:

• The admin will not be able to grant test_user an instructor on the entire demo organization, the admin will have to grant it course by course. This is because CourseAccessRole will not allow two similar records like [user: test_user, course_id: "", role: instructor, org: demo] and [user: test_user, course_id: "", role: instructor, org: Demo]. The key constraint treats org as case-insensitive

After the fix:

• Adding a single record like [user: test_user, course_id: "", role: instructor, org: demo], [user: test_user, course_id: "", role: instructor, org: Demo], or even [user: test_user, course_id: "", role: instructor, org: DEMO] will make test_user an instructor on all courses as expected

Deadline

Other information

• This is an upstream issue. We'll contribute the fix to upstream once we have a code review here