abstracts height rule to apply to the capa check, show, save actions

(cherry picked from commit 050846af)

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

@@ -551,16 +551,16 @@ section.problem {
   section.action {
     margin-top: 20px;
 
-    input.save {
-      @extend .blue-button;
+    .save, .check, .show {
+      height: ($baseline*2);
+      font-weight: 600;
     }
 
-    .check {
-      height: ($baseline*2);
+    .save {
+      @extend .blue-button;
     }
 
     .show {
-      height: ($baseline*2);
 
       .show-label {
         font-size: 1.0em;