Change font-size attributes in conversations (inbox)

refs: CNVS-25278

Test Plan:
- This is a 1 to 1 change. Font size will just be relative instead of
fixed sized (ie, rem instead of px)
- CSS should compile successfully
- Go to conversations and make sure you have none selected
- Giant envelope icon should look the same http://cl.ly/3V0y0A0T0g2L

Change-Id: Ia108dfa5efee8e1e8940e528fb7395b4ec059d0b
Reviewed-on: https://gerrit.instructure.com/72590
Tested-by: Jenkins
Reviewed-by: Chris Hart <chart@instructure.com>
Product-Review: Chris Hart <chart@instructure.com>
QA-Review: Myller de Araujo <myller@instructure.com>

Author: Colleen Palmer

app/stylesheets/pages/conversations/_conversations_new.scss

@@ -446,7 +446,7 @@ h2.subject {
       width: 100%;
 
       &:before {
-        font-size: 120px;
+        @include fontSize(120px);
       }
     }
   }