fix avatars changing width on long comments

Avatars in comments in SpeedGrader could shrink if the comment text
was too long. This sets a constant size.

Test Plan
- Enable avatars (Account settings)
- Create an assignment
- Launch SpeedGrader as a teacher.
- Create a comment.
- Create a really long comment.
- Notice that the avatars are circular and constantly sized.
- Resize the window and verify the avatars continue to look circular
  and constantly sized.

fixes GRADE-1836

Change-Id: Ic155233ca1354c3839138b3b32b06aabf1c9b752
Reviewed-on: https://gerrit.instructure.com/173431
Tested-by: Jenkins
Reviewed-by: Jeremy Neander <jneander@instructure.com>
Reviewed-by: Keith Garner <kgarner@instructure.com>
QA-Review: Jeremy Neander <jneander@instructure.com>
Product-Review: Sidharth Oberoi <soberoi@instructure.com>

app/stylesheets/bundles/speed_grader.scss

@@ -509,10 +509,11 @@ $icon-size: 1.4rem;
   overflow-y: auto;
   .comment {
     .avatar {
+      flex-grow: 0;
+      flex-shrink: 0;
       height: 24px;
-      max-width: 24px;
-      flex-shrink: 5;
       margin-#{direction(right)}: 12px;
+      width: 24px;
     }
   }
 }