refactor(templates): Dynamic image loading

static/templates/contract/de.html

@@ -27,7 +27,7 @@
 <body class="A5 landscape">
   {{ range .Runners }}
   <div class="sheet">
-    <img id="header_img" width="100%" src="data:image/png;base64,{{ $.SponsoringHeader}}" />
+    <img id="header_img" width="100%" src="data:image/png;base64,{{ loadImage "sponsoringheader" }}" />
     <div style=" padding: 0 1rem 0 1rem;">
       <div class="columns">
         <div class="column is-10">