Add custom file upload component

The default browser file upload control is difficult to style, but looks totally
out of place.

This commit replaces it with one that has a GOV.UK style button, as a first
step.

Based heavily on this example:
http://tympanus.net/codrops/2015/09/15/styling-customizing-file-inputs-smart-way/

app/templates/components/file-upload.html

@@ -0,0 +1,24 @@
+{% macro file_upload(field) %}
+  <div class="form-group{% if field.errors %} error{% endif %}" data-module="file-upload">
+    <label class="file-upload-label" for="{{ field.name }}">
+      {{ field.label }}
+      {% if hint %}
+        <span class="form-hint">
+          {{ hint }}
+        </span>
+      {% endif %}
+      {% if field.errors %}
+        <span class="error-message">
+          {{ field.errors[0] }}
+        </span>
+      {% endif %}
+    </label>
+    {{ field(**{
+      'class': 'file-upload-field'
+    }) }}
+    <label class="file-upload-button" for="{{ field.name }}">
+      Choose file
+    </label>
+    <label class="file-upload-filename" for="{{ field.name }}"></label>
+  </div>
+{% endmacro %}