Make a function for parsing help query param

Our templates are a littered with `request.args.get('help', '0')`.
This commit refactors these into a single helper method, which can be
used by the view functions, then passed to the template.

This makes the templates cleaner, and should make it easier to refactor
`help` out of the query parameters entirely in the future.

app/templates/views/send-test.html

@@ -55,7 +55,7 @@
     {% endcall %}
 
     {{ page_footer("Preview", back_link=(
-      url_for('.choose_template', service_id=current_service.id, template_type=template.template_type)) if not request.args['help'] else None
+      url_for('.send_messages', service_id=current_service.id, template_id=template.id)) if not help else None
     ) }}
   </form>