Fixed #17085, #24783 -- Refactored template library registration.

* 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.

tests/template_tests/syntax_tests/test_static.py

@@ -7,6 +7,7 @@ from ..utils import setup
 
 @override_settings(MEDIA_URL="/media/", STATIC_URL="/static/")
 class StaticTagTests(SimpleTestCase):
+    libraries = {'static': 'django.templatetags.static'}
 
     @setup({'static-prefixtag01': '{% load static %}{% get_static_prefix %}'})
     def test_static_prefixtag01(self):