Fix duplicate labels on form inputs

It’s invalid HTML to have multiple labels nested within each other. This
was happening by accident because WTForms tries to be clever – when you
put `{{ field.label }}` in a template it prints a `<label>` tag for you,
not just the text of the label. But we put our own `<label>` tags in the
HTML to have more control of them.

This commit stops WTForms being so clever.

app/templates/components/file-upload.html

@@ -1,7 +1,7 @@
 {% macro file_upload(field, button_text="Choose file", alternate_link=None, alternate_link_text=None) %}
   <form method="post" enctype="multipart/form-data" class="{% if field.errors %}error{% endif %}" data-module="file-upload">
     <label class="file-upload-label" for="{{ field.name }}">
-      <span class="visually-hidden">{{ field.label }}</span>
+      <span class="visually-hidden">{{ field.label.text }}</span>
       {% if hint %}
         <span class="form-hint">
           {{ hint }}