Update cancel URL handling in HTTP and timer function templates

- Modify header template to use explicit `cancel_url` parameter
- Add `cancel_url` to new timer and HTTP function templates
- Remove default URL fallback in header template

templates/dashboard/timer_functions/new.html

@@ -9,7 +9,7 @@ show_refresh=False,
 show_logs=False,
 show_client=False,
 show_link=False,
-dashboardUrl=url_for('timer.overview'),
+cancel_url=url_for('timer.overview'),
 title='New Timer Function')
 }}