fix gradebook assignment group and student loading race conditions
fix gradebook assignment group and student loading race conditions

test plan:
* create a course with over 10 assignment groups
* add a couple students to a course not in
 alphabetical order
* the gradebook should load and sort the users
 fully consistently (reload several times to be sure)

closes #CNVS-24052

Change-Id: I59c5534cb89da1d66d4e0b4f1673340c098b38f1
Reviewed-on: https://gerrit.instructure.com/65305
Reviewed-by: Keith T. Garner <a1a80143a840c140f31c3495ebbd9ec906e09fed@instructure.com>
Reviewed-by: Jeremy Stanley <b3f594e10a9edcf5413cf1190121d45078c62290@instructure.com>
Tested-by: Jenkins
QA-Review: Jahnavi Yetukuri <ed2b2672ccc2e59e56c0615df7f01b579202e014@instructure.com>
Product-Review: James Williams  <3a5f4ed991dbfa5c9ef76ad729d240ca12f62180@instructure.com>

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