Looked up the default template engine in the list of all engines.
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.