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_invalid_string.py

@@ -4,6 +4,7 @@ from ..utils import setup
 
 
 class InvalidStringTests(SimpleTestCase):
+    libraries = {'i18n': 'django.templatetags.i18n'}
 
     @setup({'invalidstr01': '{{ var|default:"Foo" }}'})
     def test_invalidstr01(self):