set minimum CR for submission comment name

the color property was changed to darker colors for
the comment in submission detail page and the
submission side bar.

fixes EVAL-1842
flag=none

test plan:
- enable feature flag 'High Contrast UI'.
- create and publish a course with one teacher and
  one student.
- create and publish an assignment.
- as the student, submit to the assignment.
- as the teacher, add a comment to the submission.
- as the student, view the assignment page and
  notice that the date of the comment and name of the
  commenter have atleast 4.5:1 of contrast ratio.
- as the student, view the submission details page
  and notice that the date of the comment and name of
  the commenter have atleast 4.5:1 of contrast ratio.

Change-Id: Ifc86442b9383fb8a9699146703976bf635da3656
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/270393
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Reviewed-by: Aaron Shafovaloff <ashafovaloff@instructure.com>
Reviewed-by: Syed Hussain <shussain@instructure.com>
Reviewed-by: Adrian Packel <apackel@instructure.com>
QA-Review: Aaron Shafovaloff <ashafovaloff@instructure.com>
Product-Review: Syed Hussain <shussain@instructure.com>

app/stylesheets/bundles/submission.scss

@@ -107,6 +107,14 @@
     background-color: $ic-bg-light-neutral;
     @include breakpoint(desktop) { border-#{direction(left)}: 1px solid $ic-border-light; }
   }
+
+  .comment_author_and_date {
+    @if $use_high_contrast {
+      color: #6D696A;
+    } @else {
+      color: #888888;
+    }
+  }
 }
 
 .file-upload-submission {

app/stylesheets/pages/assignments/_assignments.scss

@@ -221,7 +221,11 @@
         .comment {
           margin-#{direction(left)}: 10px;
           .signature {
+            @if $use_high_contrast {
+              color: #737373;
+            } @else {
               color: #888888;
+            }
             font-size: 0.8em;
             text-align: direction(right);
           }

app/views/assignments/_submission_sidebar.html.erb

@@ -137,7 +137,7 @@
                 </div>
               <% end %>
             </div>
-            <div class="signature" style="font-size: 0.8em; text-align: <%= direction('right') %>; color: #888;">
+            <div class="signature" style="font-size: 0.8em; text-align: <%= direction('right') %>;">
               <%= t :comment_signature, "%{author}, %{created_at}", {
                 :author => comment_author_name_for(comment),
                 :created_at => datetime_string(comment.created_at)

app/views/submissions/show.html.erb

@@ -341,7 +341,7 @@
                     </div>
                   <% end %>
                 </div>
-                <div style="text-align: <%= direction('right') %>; font-size: 0.8em; color: #888;">
+                <div class="comment_author_and_date" style="text-align: <%= direction('right') %>; font-size: 0.8em;">
                   <span class="author_name">
                     <%= author_name %>
                   </span>,