Removed some uses of global APIs from django.template.loader.
Passed a reference to the current engine when instanciating Template.
Isolated template tests from Django settings.
Looked up the default template engine in the list of all engines.

As the default engine now depends on TEMPLATES, the entire test suite
must be adapter wherever it relied on the TEMPLATE_* settings.
Supported multiple template engines in get_template and select_template.
Added support for explicitly selecting an engine.
Removed private API find_template.
Removed private API get_template_from_string.
Removed extraneous arguments in Engine.from_string.

This aligns the Django Template Engine API with the common template
backend API.
Supported multiple template engines in render_to_string.

Adjusted its API through a deprecation path according to the DEP.
Added a comment about the last use of Engine.get_default().
Moved context_processors from django.core to django.template.