Put button to add content next to its related form

This way the relationship between the two elements is more obvious. And
since now the button and the form are siblings, it's easier to find one
based on the other using CSS or JavaScript.

app/assets/stylesheets/layout.scss

@@ -2462,6 +2462,7 @@ table {
 
 .add-related-content {
   display: block;
+  margin-bottom: $line-height;
 
   @include breakpoint(medium) {
     float: right;