Add TODO to remove button padding override used on GOV.UK

With the toolkits bump it includes updates to buttons which improve
the padding on buttons, GOV.UK already includes overrides to the previous
padding of buttons so this commit adds a note to remove this as part of
a wider review in favour of the padding now used in the toolkit

app/assets/stylesheets/helpers/_buttons.scss

@@ -12,6 +12,10 @@
 
 .button,
 .button.medium {
+  /*
+   * TODO: Remove button padding override, upstream (govuk_frontend_toolkit 4.12.0)
+   * has included better improvements to padding.
+   */
   padding: 0.3em 0.6em 0.2em 0.6em;
 }
 
@@ -75,7 +79,7 @@
 /* get started buttons */
 .get-started {
   margin:1.5em 0 0 0;
-  
+
   .destination {
     @include core-14;
     color: $text-colour;
@@ -98,7 +102,7 @@ input{
     min-width: 8em;
     margin: 0 0.5em;
     padding: 0.25em;
-  
+
     @include ie-lte(7) {
       width:8em;
     }
@@ -117,4 +121,3 @@ input{
     }
   }
 }
-