Add xref helper utils template

tailbone_harvest/templates/harvest-util.mako

@@ -0,0 +1,23 @@
+## -*- coding: utf-8; -*-
+
+<%def name="render_xref_buttons()">
+  <b-button type="is-primary"
+            % if harvest_url:
+            tag="a" href="${harvest_url}" target="_blank"
+            % else:
+            disabled title="${harvest_why_no_url}"
+            % endif
+            icon-pack="fas"
+            icon-left="external-link-alt">
+    View in Harvest
+  </b-button>
+</%def>
+
+<%def name="render_xref_helper()">
+  <nav class="panel">
+    <p class="panel-heading">Cross-Reference</p>
+    <div class="panel-block buttons">
+      ${self.render_xref_buttons()}
+    </div>
+  </nav>
+</%def>