pages: global url_map modification fix

* Restores the url_map after the request to avoid affecting
  subsequent requests.

Signed-off-by: Javier Martin Montull <javier.martin.montull@cern.ch>
please check https://github.com/mitsuhiko/flask/blob/6fcc2ae3f4536e4d2651c1a00abe2306212a5bfd/flask/ctx.py#L43