Fix the cohort average grade calculation in the MobileUsersCoursesGrades endpoint

A cohort in this context is the group of useres enrolled in the course, not the technical meaning of cohort we attribute in the codebase. This fixes the grades average calculation to include every user enrolled in the course, rather than just the course's cohort.

*JIRA Ticket*: [OC-2886](https://tasks.opencraft.com/browse/OC-2886)

*Testing instructions*:
1. Set up OAuth client in LMS admin if you don't already have one set up.
2. Obtain OAuth access token for your test user: curl -X POST -d "client_id={client_id}&client_secret={client_secret}&grant_type=password&username={username}&password={password}" https://lms.apros.local/oauth2/access_token/
3. Verify that you can access the api with your OAuth access token: curl 'http://lms.mcka.local/api/server/mobile/v1/users/courses/grades?username={username}&course_id={course_id} -H 'Authorization: Bearer {access_token}' -v
4. Make sure the user grades are returned and that their course average grade is calculated correctly.