remove excess queries for quiz csv student analysis report
remove excess queries for quiz csv student analysis report

Removes the look up for a quiz and its context each time by simply
saving a reference to the context since all the submissions should be
for the same quiz (and thus same context).

test plan:
  - Make sure the Quiz Statistics CSV still functions correctly

Change-Id: I401560bff993d2de4ec1bed80f4ad013f223b21b
Reviewed-on: https://gerrit.instructure.com/21968
Reviewed-by: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
QA-Review: Myller de Araujo <e5c7c5e1b7f2ce5e9870cc98ec52b20b3006cb48@instructure.com>
Product-Review: Stanley Stuart <2b43fb8b7a234825d50dd49ce7892d78a59da8f3@instructure.com>

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