Note that the cookie is not changed every request, just the token retrieved
by the `get_token()` method (used also by the `{% csrf_token %}` tag).
While at it, made token validation strict: Where, before, any length was
accepted and non-ASCII chars were ignored, we now treat anything other than
`[A-Za-z0-9]{64}` as invalid (except for 32-char tokens, which, for
backwards-compatibility, are accepted and replaced by 64-char ones).
Thanks Trac user patrys for reporting, github user adambrenecki
for initial patch, Tim Graham for help, and Curtis Maloney,
Collin Anderson, Florian Apolloner, Markus Holtermann & Jon Dufresne
for reviews.
`action="."` strips query parameters from the URL which is not usually what
you want. Copy-paste coding of these examples could lead to difficult to
track down bugs or even data loss if the query parameter was meant to alter
the scope of a form's POST request.
Added the CSRF_TRUSTED_ORIGINS setting which contains a list of other
domains that are included during the CSRF Referer header verification
for secure (HTTPS) requests.
