Fix missing CSRF token on send SMS page

As part of https://github.com/alphagov/notifications-admin/pull/187 the file
upload pattern was changed to auto-submit once a file had been picked. The
form that was submitted was, however, missing a CSRF token, as well as a submit
button for non-JS users.

This commit makes the file upload pattern self-contained, so that it will always
include a form with a CSRF token in a hidden input and a submit button, which is
then hidden when Javascript loads.

app/templates/components/file-upload.html

@@ -1,5 +1,5 @@
 {% macro file_upload(field, button_text="Choose file") %}
-  <div class="form-group{% if field.errors %} error{% endif %}" data-module="file-upload">
+  <form method="post" enctype="multipart/form-data" class="form-group{% if field.errors %} error{% endif %}" data-module="file-upload">
     <label class="file-upload-label" for="{{ field.name }}">
       <span class="visually-hidden">{{ field.label }}</span>
       {% if hint %}
@@ -20,5 +20,7 @@
       {{ button_text }}
     </label>
     <label class="file-upload-filename" for="{{ field.name }}"></label>
-  </div>
+    <input type="hidden" name="csrf_token" value="{{ csrf_token() }}" />
+    <input type="submit" class="file-upload-submit" value="Submit" />
+  </form>
 {% endmacro %}