Add dragscroll library back to base HTML template

We lost this as well as the associated functionality when we moved over to a SPA in

listenbrainz/webserver/templates/index.html

@@ -68,6 +68,7 @@
 
 <script src="{{ url_for('static', filename='js/lib/jquery-2.1.0.min.js') }}"></script>
 <script src="{{ url_for('static', filename='js/lib/bootstrap.min.js') }}"></script>
+<script src="{{ url_for('static', filename='js/lib/dragscroll.js') }}"></script>
 <script id="global-react-props" type="application/json">{{ global_props|safe }}</script>
 <script id="page-react-props" type="application/json">{{ props|safe }}</script>
 <script src="{{ get_static_path('indexPage.js') }}" type="text/javascript"></script>