syncs up font-weight across problem button/input actions

(cherry picked from commit 14100ba1)

common/lib/xmodule/xmodule/css/capa/display.scss

@@ -553,7 +553,7 @@ section.problem {
 
     .save, .check, .show {
       height: ($baseline*2);
-      font-weight: 500;
+      font-weight: 600;
       vertical-align: middle;
     }
 
@@ -565,7 +565,7 @@ section.problem {
 
       .show-label {
         font-size: 1.0em;
-        font-weight: 500;
+        font-weight: 600;
       }
     }