ensure that document matches student in speedgrader

fixes CNVS-1739

ajax calls to get the document were coming in out of order -
this fix makes sure that the student id of the response
matches the current student id

test plan:
  - As teacher make assignment that allows document submissions
  - As 5-10 students, submit pdfs/word documents for the assignment
  (no need to redo the above steps if you already have this setup somewhere)
  - In speedgrader try to click very quickly and get the wrong document to
    appear for a student (hint: this is easier with larger documents)
     - Do this with Crocodoc previews, Google Doc previews, and Scribd previews
     (each has a slightly different fix, so it is important to test each)
     - You shouldn't be able to get the wrong assignment to show up

Change-Id: Icac0a6d94f1ed7a73adbbc6fbcb47d84ff8e349d
Reviewed-on: https://gerrit.instructure.com/28300
QA-Review: Amber Taniuchi <amber@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
Product-Review: Mike Nomitch <mnomitch@instructure.com>