Fix tabnav styles on #create and #edit views

The Primer CSS-provided `tabnav` styles were not being used on the Edit
and Preview tabs on the create and edit pages.

After following Primer's documentation [1], it looks like we were using
the `aria-current` attribute incorrectly. Dynamically adding/removing
this attribute on the selected tab fixes the issue.

[1]: https://primer-css-git-next-inputs.primer.now.sh/css/components/navigation#tabnav

lib/gollum/public/gollum/javascript/gollum.js.erb

@@ -380,8 +380,12 @@ $(document).ready(function() {
       active_tab = '#edit.tabnav-tab';
     }
 
+    $('.tabnav-tab.selected').removeAttr('aria-current');
     $('.tabnav-tab.selected').removeClass('selected');
+
+    $(active_tab).attr('aria-current', 'page');
     $(active_tab).addClass('selected');
+
     $('.tabnav-div').hide();
     $(active_div).show();
   }

lib/gollum/templates/create.mustache

@@ -4,13 +4,15 @@
   <h1 class="py-4">Create New Page</h1>
 </div>
 <div id="wiki-content" class="create edit">
-	<div class="tabnav">
-	  <nav class="tabnav-tabs" aria-label="Foo bar">
-	    <a href="#" id="edit" class="tabnav-tab selected" aria-current="edit">Edit</a>
-	    <a href="#" id="preview" class="tabnav-tab" aria-current="preview">Preview</a>
-	  </nav>
-	</div>
-	
+  <div class="tabnav">
+    <nav class="tabnav-tabs" aria-label="Toggle edit or preview mode">
+      <a href="#" id="edit" class="tabnav-tab selected" aria-current="page">
+        Edit
+      </a>
+      <a href="#" id="preview" class="tabnav-tab">Preview</a>
+    </nav>
+  </div>
+
   <div class="has-sidebar tabnav-div" id="edit-content">
     {{>editor}}
   </div>

lib/gollum/templates/edit.mustache

@@ -5,8 +5,10 @@
 </div>
 <div class="tabnav">
   <nav class="tabnav-tabs">
-    <a href="#" id="edit" class="tabnav-tab selected" aria-current="edit">Edit</a>
-    <a href="#" id="preview" class="tabnav-tab" aria-current="preview">Preview</a>
+    <a href="#" id="edit" class="tabnav-tab selected" aria-current="page">
+      Edit
+    </a>
+    <a href="#" id="preview" class="tabnav-tab">Preview</a>
   </nav>
 </div>
   <div class="tabnav-div" id="edit-content">{{>editor}}</div>