Fix Sorting by ForeignKey

Removed do_inline_sorting_url and get_object_or_404 check in admin. The model_type_id should *always* be passed in.
Removed test that asserted that Categories weren't sortable as part of the Project admin. Categories *should* be sortable as part of Project admin as they are a Sortable ForeignKey.
Fixed object_rep template to pass in model_type_id again.
Updated README.
Version bump to 2.0.21

adminsortable/templates/adminsortable/shared/object_rep.html

@@ -2,5 +2,5 @@
 
 <form>
     <input name="pk" type="hidden" value="{{ object.pk }}" />
-    <a href="{% url opts|admin_urlname:'do_sorting' %}" class="admin_sorting_url"><i class="fa fa-{% if forloop.first %}sort-desc{% elif forloop.last %}sort-asc{% else %}sort{% endif %}"></i> {{ object }}</a>
+    <a href="{% url opts|admin_urlname:'do_sorting' object.model_type_id %}" class="admin_sorting_url"><i class="fa fa-{% if forloop.first %}sort-desc{% elif forloop.last %}sort-asc{% else %}sort{% endif %}"></i> {{ object }}</a>
 </form>