Remove deprecated validator

Thanks to m@maikhoepfel.de who submitted this patch via the mailing
list.
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
Support Django's 1.5 AUTH_USER_MODEL feature
Switch to new {% url %} tag style.

Made easy thanks to django-future-url. Resolved two non-ASCII issue and
removed one BOM.
Fix verify_exists for ExtendedURLValidator

ExtendedURLValidator does not pass down verify_exist in it's __init__
method. But as it's super class URLValidator then sets self.verify_exists to
False, any passed value was ignored.
Allow and test against Django 1.5