increase 10px font size throughout react_gradebook to 12px min

Refs: CNVS-26581

test plan:
- as a teacher using the old or new ui
- navigate to a course that has users that have submitted assignments
  that have been graded so you can view gradebook
- click the grades sub-nav link
- from the gradebook view the following have been updated
  note: screen shots are from orig gradebook but look the same:
  - ko accessibility warning: http://screencast.com/t/2q5qqEqwfes
  - student section: http://screencast.com/t/sNQcjOecb4
  - secondary identifier: http://screencast.com/t/9BwHyVcbJI2C
  - slick header column: http://screencast.com/t/rp1InVw4Qmx3
  - assignment-points-possible: http://screencast.com/t/FAYK4WHMLO2
  - letter-grade-points: http://screencast.com/t/ZNDLWnbS

Change-Id: I7f51a2cecc9db0a8300dfbf9682120f653ddbb12
Reviewed-on: https://gerrit.instructure.com/73188
Reviewed-by: Colleen Palmer <colleen@instructure.com>
Product-Review: Colleen Palmer <colleen@instructure.com>
Tested-by: Jenkins
QA-Review: Myller de Araujo <myller@instructure.com>

app/stylesheets/pages/react_gradebook/_react_gradebook.scss

@@ -26,7 +26,7 @@ $gradebook_resubmitted-color: desaturate(darken($ic-bg-light-alert, 7), 2);
 
 a#accessibility_warning {
   @include accessibility-prompt;
-  @include fontSize($ic-font-size--small);
+  @include fontSize(14px);
 }
 
 #footer {
@@ -74,12 +74,12 @@ $cell_height: 33px;
 
 .student-section {
   margin-top: -7px;
-  @include fontSize(10px);
+  @include fontSize(12px);
 }
 
 .secondary_identifier_cell, .custom_column {
   color: #333333;
-  @include fontSize(10px);
+  @include fontSize(12px);
   text-align: center;
   line-height: 35px;
 }
@@ -101,7 +101,7 @@ $cell_height: 33px;
   background-color: #f3f4f8;
   background-image: none;
   padding: 10px;
-  @include fontSize($ic-font-size--xsmall);
+  @include fontSize(12px);
   text-align: center;
   font-weight: normal;
   // override jqueryUI style
@@ -134,7 +134,7 @@ $cell_height: 33px;
   .assignment-due-date {
     position: relative;
     bottom: 4px;
-    @include fontSize(10px);
+    @include fontSize(12px);
     // If high contrast, default to regular dark text
     @if $use_high_contrast == false { color: lighten($ic-font-color-dark, 15%); }
   }
@@ -300,7 +300,7 @@ $cell_height: 33px;
     border: none;
     text-align: center;
     outline: none;
-    @include fontSize($ic-font-size--xsmall);
+    @include fontSize(12px);
     width: 100%;
     padding: 0;
     margin: 0;
@@ -492,7 +492,7 @@ $gradebook_checkbox_width: 16px;
 .letter-grade-points,
 .gpa-scale-points {
   position: absolute;
-  @include fontSize(11px);
+  @include fontSize(12px);
   padding-left: 8px;
   line-height: 19px;
   color: $gradebook_letter-grade-font;