use mark_safe for reorder div

I encountered a problem when updating to django 2.0
Using the sortableadminmixin.

The div was rendered with > and so on instead of proper html tags.
By adding mark_safe to the div generation function all was working again.

Author: hiaselhans

adminsortable2/admin.py

@@ -8,6 +8,7 @@
 
 from django import forms
 from django.utils.translation import ugettext_lazy as _
+from django.utils.safestring import mark_safe
 from django.conf.urls import url
 from django.core.exceptions import ImproperlyConfigured
 from django.core.paginator import EmptyPage
@@ -165,7 +166,7 @@ def func(this, item):
             html = ''
             if this.enable_sorting:
                 html = '<div class="drag js-reorder-{1}" order="{0}">&nbsp;</div>'.format(getattr(item, this.default_order_field), item.pk)
-            return html
+            return mark_safe(html)
 
         setattr(func, 'allow_tags', True)
         # if the field used for ordering has a verbose name use it, otherwise default to "Sort"