Use <button> instead of <a> without href

It is more semantic to use a `<button>` tag in the place of an `<a>`
when there is no other page being linked to. In this circumstance, we're
using JavaScript to present a modal to the user on click.

This change makes the "Upload" and "Rename" buttons appear in the
browser's tab index.

lib/gollum/templates/navbar.mustache

@@ -54,21 +54,21 @@
 
       {{#allow_editing}}
         {{#allow_uploads}}
-          <a
+          <button
             class="btn BtnGroup-item btn-sm hide-sm hide-md
               minibutton-upload-page"
           >
             Upload
-          </a>
+          </button>
         {{/allow_uploads}}
 
         {{#editable}}
-          <a
+          <button
             class="btn BtnGroup-item btn-sm hide-sm hide-md
               minibutton-rename-page"
           >
             Rename
-          </a>
+          </button>
           <a
             class="btn BtnGroup-item btn-sm hide-sm hide-md"
             href="{{edit_path}}/{{escaped_url_path}}"
@@ -81,7 +81,6 @@
     </div>
   </div>
 
-
   {{#allow_editing}}
     {{#editable}}
       <div class="TableObject-item">