Update {% url %} syntax to Django 1.5 style.
Fix for Django's dropped verify_exists functionality

Please note that ExtendedURLFields's verify_exist does not
default to True anymore.
ExtendedURLValidator: don't pass down verify_exists.
Rework ExtendedURLValidator and tests

* Tidy implementation of validator
* Tests improved and moved to tests/integration (as they call resolve
  internally)
Add integration test for ExtendedURLField
Ensure promotions work correct with modified URL field
Rework validation that local URLs don't already exist

This is required to allow the pages component of the dashboard to
continue to work correctly.
Add debug logging filter to sandbox

This removes a deprecation warning related to:
https://docs.djangoproject.com/en/dev/releases/1.4/#request-exceptions-are-now-always-logged