added no scores message to assignment details max/min/ave

fixes CNVS-9709

infinity no longer shows up as max/min score when no submissions are graded
message added to average score when no submissions are graded

Test Plan:
  - as teacher go to gradebook2
  - click assignment_details on assignment w/ no grades
  - max/min/ave scores should say "No graded submissions"
  - assignments with grades should work as usual

Change-Id: Ib7574fe8745f63710b1a05e6c5f44e5d3be96522
Reviewed-on: https://gerrit.instructure.com/27965
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>

app/coffeescripts/AssignmentDetailsDialog.coffee

@@ -1,9 +1,10 @@
 define [
+  'i18n!assignment_details'
   'jquery'
   'jst/AssignmentDetailsDialog'
   'jqueryui/dialog'
   'compiled/jquery/fixDialogButtons'
-], ($, assignmentDetailsDialogTemplate) ->
+], (I18n, $, assignmentDetailsDialogTemplate) ->
 
   class AssignmentDetailsDialog
     constructor: ({@assignment, @students}) ->
@@ -35,14 +36,18 @@ define [
       locals =
         assignment: assignment
         cnt: scores.length
-        max: Math.max scores...
-        min: Math.min scores...
-        average: do (scores) ->
+        max: @nonNumericGuard Math.max scores...
+        min: @nonNumericGuard Math.min scores...
+        average: do (scores) =>
           total = 0
           total += score for score in scores
-          Math.round(total / scores.length)
+          @nonNumericGuard Math.round(total / scores.length)
 
       scores: scores
       locals: locals
 
-
+    nonNumericGuard: (number) =>
+      if isFinite(number) and not isNaN(number)
+        number
+      else
+        I18n.t('no_graded_submissions', "No graded submissions")

app/coffeescripts/ember/screenreader_gradebook/controllers/screenreader_gradebook_controller.coffee

@@ -134,7 +134,7 @@ define [
 
     assignmentDetails: (->
       return null unless @get('selectedAssignment')?
-      {locals} = AssignmentDetailsDialog::compute.call null, {
+      {locals} = AssignmentDetailsDialog::compute.call AssignmentDetailsDialog::, {
         students: @studentsHash()
         assignment: @get('selectedAssignment')
       }