Adjust padding and presentation of mobile menu

To give mobile page headers more structure, this commit adjusts the
padding to be more symmetrical. It also adds a `border-bottom` on mobile
only. In my opinion this improves readability by telling the reader,
"This is where the navigation menu ends and the page content begins."

lib/gollum/templates/navbar.mustache

@@ -1,4 +1,11 @@
-<nav class="TableObject actions pt-4 px-2 px-lg-0 overflow-x-scroll">
+<nav class="TableObject
+            actions
+            border-bottom
+            border-md-0
+            p-2
+            pt-lg-4
+            px-lg-0
+            overflow-x-scroll">
   <div class="TableObject-item hide-lg hide-xl">
     {{>mobilenav}}
   </div>