Display and overwrite task order uploads correctly.

- Populate the POSTed form with additional data from the existing TO
  model. This way the pdf attachment has to be explicitly overwritten.
- Adjust the frontend so that if there is an existing PDF, it only sends
  a file input back if the user removes the existing PDF.

templates/components/upload_input.html

@@ -20,7 +20,7 @@
         {{ field.label }}
       {% endif %}
       {{ field.description }}
-      <div class="upload-widget">
+      <div v-if="!hideInput" class="upload-widget">
         <label class="upload-label" for="{{ field.name }}">
           <span class="upload-button">
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