Fixed #20869 -- made CSRF tokens change every request by salt-encrypting them

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.

docs/topics/security.txt

@@ -65,10 +65,10 @@ this if you know what you are doing. There are other :ref:`limitations
 <csrf-limitations>` if your site has subdomains that are outside of your
 control.
 
-:ref:`CSRF protection works <how-csrf-works>` by checking for a nonce in each
+:ref:`CSRF protection works <how-csrf-works>` by checking for a secret in each
 POST request. This ensures that a malicious user cannot simply "replay" a form
 POST to your website and have another logged in user unwittingly submit that
-form. The malicious user would have to know the nonce, which is user specific
+form. The malicious user would have to know the secret, which is user specific
 (using a cookie).
 
 When deployed with :ref:`HTTPS <security-recommendation-ssl>`,