Implemented async barcode generation using async helpers

ref #13

src/templates/sponsoring_contract.html

@@ -54,7 +54,7 @@
           </div>
         </div>
         <div class="column">
-          <img width="100%" style="vertical-align: revert; margin-top: auto;" src="{{--bc this.id}}" />
+          <img width="100%" style="vertical-align: revert; margin-top: auto;" src="{{--bc this.id 'code39'}}" />
         </div>
       </div>
       <div class="columns" style="padding-top: 1rem;">