Override "submit_buttons_top" section to prevent KeyError in context (#710)

This block is rendered conditionally. Will not appear if save_on_top = 
False

Author: john-parton

simple_history/templates/simple_history/object_history_form.html

@@ -13,6 +13,10 @@
   </div>
 {% endblock %}
 
+{% block submit_buttons_top %}
+  {% include "simple_history/submit_line.html" %}
+{% endblock %}
+
 {% block submit_buttons_bottom %}
   {% include "simple_history/submit_line.html" %}
 {% endblock %}