Display "Multiple Due Dates" on speedgrader when applicable. Fixes #CNVS-740
Display "Multiple Due Dates" on speedgrader when applicable. Fixes #CNVS-740

On SpeedGrader, when an assignment has multiple due dates, in the header,
display something to let the grader know that.

Testing Notes:
==========
* Using an assignment with no due date, visit SpeedGrader and verify
   that it says, "due: No Due Date"
* Using an assignment with a single (no override) due date, visit
   SpeedGrader and verify that it says "due: <the date>"
* Using an assignment with multiple override due dates, visit
   SpeedGrader and verify that is says "due: Multiple Due Dates"

NOTE: This also changes the previous behavior of not showing anything for a
due date when the assignment didn't have one to now showing
"due: No Due Date" when there is no due date. This is because the assignment
might have no due date but an override might specify one.

Change-Id: I1337015eef623f9579be3af990c8651cacd957af
Reviewed-on: https://gerrit.instructure.com/16391
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
Reviewed-by: Zach Pendleton <514dbb4d54d4f29e454990df7d075678f43f92aa@instructure.com>
QA-Review: Marc LeGendre <d33c80bc45d65303e33ca83108a9952b745af9ef@instructure.com>

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