[V2] Enrollment Tracking Backend Schemas, Endpoints

[mraysu]

• Create the Enrollment object Schema
• Update Section and Student Schemas to remove embedded enrollment lists
• Create the following endpoints:
  • GET /api/students/:id/enrollment: Retrieve student enrollment objects
    • Admin users can retrieve all enrollments, teachers can only retrieve enrollments for their assigned courses
  • GET /api/sections/:id/enrollment: Retrieve section enrollment objects
    • Admin users can retrieve all enrollments, teachers can only retrieve enrollments for their assigned courses
  • POST /api/enrollment: Insert enrollment object
    • Requires admin privilege, teachers cannot create new enrollments
  • PATCH /api/enrollment/:id: Update enrollment object
    • Teachers can update pre and post assessment scores. Admins can also update status. Section and Student fields are immutable
• Integrate the following handlers:
  • When students are enrolled in a new section, create a new Enrollment Object with status "ACTIVE"
  • When students are unenrolled from a section before a section ends, upsert the Enrollment Object with status "DROPPED"