Add data-tab-nav attribute to tab nav container (#130)

In wagtail 2.13, the `data-tab-nav' attribute is used for selecting
tab nav elements, replacing selection by the `tab-nav' class.

wagtailmedia/templates/wagtailmedia/chooser/chooser.html

@@ -6,7 +6,7 @@
 {{ uploadform.media.css }}
 
 {% if uploadform %}
-    <ul class="tab-nav merged">
+    <ul class="tab-nav merged" data-tab-nav>
         <li class="{% if not uploadform.errors %}active{% endif %}"><a href="#search" >{% trans "Search" %}</a></li>
         <li class="{% if uploadform.errors and media_type == 'audio' %}active{% endif %}"><a href="#upload-audio">{% trans "Upload Audio" %}</a></li>
         <li class="{% if uploadform.errors and media_type == 'video' %}active{% endif %}"><a href="#upload-video">{% trans "Upload Video" %}</a></li>