move the whitespace attribute to label mixin

app/assets/stylesheets/content/_forms.sass

@@ -44,6 +44,7 @@ $form--field-types: (text-field, text-area, select, check-box, range-field, sear
   vertical-align: middle
   text-overflow: ellipsis
   overflow: hidden
+  white-space:  nowrap
 
 // A general CSS class to be applied to forms using the above defined form style.
 // We can't define this on form itself as this would break a lot of existing forms.
@@ -242,7 +243,6 @@ fieldset.form--fieldset
   @include grid-content(2)
   @include label-style
   padding:      0 1rem 0 0
-  white-space:  nowrap
   font-size:    $form-label-fontsize
   line-height:  $base-line-height
   color:        $form-label-color