Improve icons size on goals index

We're also using a percentage to separate the icons, since using the
viewport width causes strange result on screens where the element
max-width (which is based in rem) is much smaller than the size of the
window.

app/assets/stylesheets/sdg/goals/index.scss

@@ -18,13 +18,13 @@
   }
 
   .sdg-goal-list {
-    $spacing: 1vw;
+    $spacing: 1.5%;
 
     @include sdg-goal-list($spacing);
     @include grid-row;
     @include grid-column-gutter;
 
-    max-width: 40rem;
+    max-width: 60rem;
     width: 100%;
 
     li {