[Feature] External Identifier/References

Co-authored-by: Tim Lorsbach <tim@lorsba.ch>
Reviewed-on: enviPath/enviPy#139

templates/actions/objects/compound.html

@@ -11,6 +11,10 @@
         <a role="button" data-toggle="modal" data-target="#set_scenario_modal">
             <i class="glyphicon glyphicon-plus"></i> Set Scenarios</a>
     </li>
+    <li>
+        <a role="button" data-toggle="modal" data-target="#generic_set_external_reference_modal">
+            <i class="glyphicon glyphicon-plus"></i> Set External Reference</a>
+    </li>
 {% endif %}
     <li>
         <a role="button" data-toggle="modal" data-target="#generic_copy_object_modal">