Add revision history to invoices/payments.

Also patches previously introduced reversion permissions hole.

Supersedes and closes #337.

RIGS/templates/invoice_detail.html

@@ -103,5 +103,8 @@
                 </div>
             </div>
         </div>
+        <div class="col-12 text-right">
+            {% include 'partials/last_edited.html' with target="invoice_history" %}
+        </div>
     </div>
 {% endblock %}