Make the main <nav> element a TableObject

There was a display issue, where navbar items's outline styles were
being cut off due to the parent `<nav>` element's margin and padding.

Fortunately, we can do away with the navbar wrapper div entirely. It was
not doing anything important except defining its children as
`TableObject` items. But we can just do this on the `<nav>` itself.

lib/gollum/templates/navbar.mustache

@@ -1,5 +1,4 @@
-<nav class="actions pt-4 px-2 px-lg-0 overflow-x-scroll">
-  <div class="TableObject">
+<nav class="TableObject actions pt-4 px-2 px-lg-0 overflow-x-scroll">
     <div class="TableObject-item hide-lg hide-xl">
       {{>mobilenav}}
     </div>
@@ -102,5 +101,4 @@
         {{/newable}}
       {{/editable}}
     {{/allow_editing}}
-  </div>
 </nav>