Fixed #22185 -- Added settings.CSRF_COOKIE_AGE

Internet Explorer has the ability to block/disable persistent cookies, and corruption of the index.dat cache
(i.e. to disk errors) can cause Django sites to authenticate correctly but fail to do FORM POST's.  To
avoid this behavior, provide the option to configure the CSRF cookie age and set it to be None to be session-based.

Test added and documentation updated.

https://code.djangoproject.com/ticket/22185