Fixed #20550 -- Added ability to preserve test db between runs

This reverts commit cb9c9a7b.

Note that the stdlib's version of pbkdf2_hmac will also be used in the upcoming Python 2.7.8 release

Thanks apollo13 for the review.

Thanks matklad for the report.

Fixed #22682 -- `makemigrations` will create `MIGRATION_MODULES` package

`makemigrations` will automatically create the package specified
in `MIGRATION_MODULES` if it doesn't already exist.

Thanks ovidiuc4 for the report.

Making SQL management commands migration aware.

Thanks Simon Charette and Tim Graham for the reviews.

Fixed #22720 -- Migrations attempt to create _order twice.

Python 2.7.7 includes compare_digest in the hmac module, but it requires
both arguments to have the same type. This is usually not a problem on
Python 3 since everything is text, but we have mixed unicode and str on
Python 2 -- hence make sure everything is bytes before feeding it into
compare_digest.

Thanks danielsamuels for the report

While at it, fixed a problem in returning empty values
(still with BinaryField/Oracle/Python3).

Thanks Tim Graham for the report.

Thanks semenov for the report.

`django.get_version` was actually used by the module.

Thanks Aymeric Augustin for the report end the review.

This makes it possible to run django.setup() in management commands that
don't need a settings module. In addition it simplifies error handling.

Thanks Claude for the review.

`makemigrations --dry-run` will output the complete migrations file
that would be written if it's used along with `--verbosity 3`.

Added documentation about `--dry-run` and `--merge`.