prevent extra time given to students for timed quiz from disappearing
prevent extra time given to students for timed quiz from disappearing

fixes CNVS-21632

test plan:
- create a Quiz in a course with at least three students
- moderate the Quiz and give several of them extra time
- wait 5-10 minutes
- grant new (or alter existing) extensions to at least one student
- refresh the page, all existing extensions should remain visible

Change-Id: I0f2ca341c0957c6421d6755619c5c2595743cdb4
Reviewed-on: https://gerrit.instructure.com/58493
Reviewed-by: Ryan Taylor <2eafa1f6bf34395b543d6ca7f438461b0774d24f@instructure.com>
Tested-by: Jenkins
QA-Review: Michael Hargiss <59e3fde81b8d9a4f5e0cc016a3c2695591912604@instructure.com>
Product-Review: Jason Sparks <542e620f0c6c3447ce6e2e8c83a97102b99f89c6@instructure.com>

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