speed grader: only show data for selected student
speed grader: only show data for selected student

there was a bug occuring when the 'next' or 'prev' button
was clicked shortly after updating a student's grade -- the
student data being shown was for the inccorrect student. this
commit fixes that bug by checking in the AJAX callback to
make sure that the data received belongs to the
currently-selected student.

closes CNVS-16958

test plan:

1. Have a quiz with at least two students and one quiz submission
2. Update score on quiz
3. Click the next student button immediately after the tab loading icon stops
	 spinning and prior to the page finishing loading (this is how the bug was
	 reproduced. see http://screencast.com/t/hGZmNjP8KrJ6 )
4. Notice that the submission data being shown on the page is for the
	 currently-selected student. Before this fix, the data would show up
	 for the previously-selected student.
5. Also, update the score on a quiz, and don't hit the 'next' student
	 button (let the page reload). Make sure the data on the page updates
	 correctly and shows the information for the currently-selected
	 student.

Change-Id: Ibd4729416a6cb726288093fb144c1c2eda6aeacf
Reviewed-on: https://gerrit.instructure.com/46369
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
Reviewed-by: Josh Simpson <bfb4987b134a00f73d881fff5113b1d5fa1876bc@instructure.com>
QA-Review: Amber Taniuchi <ef4bfd1e6d278f6bc7d4a478f02be2cb9581a0a5@instructure.com>
Product-Review: Spencer Olson <7cc0890d6f1f70810a023e7173eca5724d6c8e49@instructure.com>

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