Thanks Shivang Bharadwaj for the initial patch.

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.

Thanks Aymeric for the initial patch and Carl for review.

Per deprecation timeline; refs a3e783fe.

Forwardport of ae1d663b
from stable/1.8.x plus more.

* Converted the ``libraries`` and ``builtins`` globals of
  ``django.template.base`` into properties of the Engine class.
* Added a public API for explicit registration of libraries and builtins.

Thanks Preston Timmons for the report.

Rendering a Jinja template with self in the context threw an error.
While self is a reserved variable in Jinja, including self in the
context is not an error, so Django should respect that.

If importing or initializing a template backend fails, attempting to
access this template backend again must raise the same exception.

A deprecation path is required because the return type of
django.template.loader.get_template changed during the
multiple template engines refactor.

test_csrf_token_in_404 was incorrect: it tested the case when the
hardcoded template was rendered, and that template doesn't depend on the
CSRF token. This commit makes it test the case when a custom template is
rendered.

Changed import style to avoid confusion between Django's and Jinja2's
APIs.

This is the expected behavior, but given RequestContext's tortuous
implementation, a straightforward use of its API results in the
opposite.

This commits fixes a regression that must have happened at different
points in the multiple templates engine refactor for different features.