Forwardport of 2456276b from stable/1.8.x

Nested deconstruction should recursively deconstruct items within list,
tuple and dict values.

QuerySet.exists() incorrectly handled query.group_by = True
case (grouping by all select fields), causing GROUP BY
expressions to be wiped along with select fields.

Thanks to Tim Graham for the report.

Added apps to unmigrated apps if the migrations module is a file
or a folder missing __init__.py.

Thanks to Ernest0x for the bug report.

Thanks Raphaël Hertzog for the report and help on the patch, and Tim Graham
for the review.

* 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.