perf(layout): improve margins for tail block (#1243)

Keeps the margin of the elements in the tail area consistent regardless of the number of siblings, or if a <script> is inserted.

_sass/addon/commons.scss

@@ -1121,7 +1121,7 @@ search {
   }
 }
 
-/* --- main wrapper --- */
+/* --- basic wrappers --- */
 
 #main-wrapper {
   position: relative;
@@ -1135,6 +1135,16 @@ search {
   @include ml-mr(0);
 }
 
+#tail-wrapper {
+  > :not(script) {
+    margin-top: 3rem;
+  }
+
+  > :only-child {
+    margin-top: 0;
+  }
+}
+
 /* --- button back-to-top --- */
 
 #back-to-top {