Explicit is better than implicit.

The new signature enables better support for routing RunPython and
RunSQL operations, especially w.r.t. reusable and third-party apps.

This commit also takes advantage of the deprecation cycle for the old
signature to remove the backward incompatibility introduced in #22583;
RunPython and RunSQL won't call allow_migrate() when when the router
has the old signature.

Thanks Aymeric Augustin and Tim Graham for helping shape up the patch.

Refs 22583.

Thanks to DavidMuller for the report.

As suggested by Anssi. This has the slightly strange side effect of
passing the expression to Expression.convert_value has the expression
passed back to it, but it allows more complex patterns of expressions.

Joint effort between myself, Josh, Anssi and Shai.

This opens more possibilities, like accessing context.template.origin.

It also follows the chain of objects instead of following a shortcut.

Workaround for http://bugs.python.org/issue20747.
In some corner cases, Python 2 inserts a newline in a header value
despite `maxlinelen` passed in Header constructor.
Thanks Tim Graham for the review.

Thanks Joris Benschop for the report, and Tim Graham for the tests.

Thanks Tim Graham for pointing out the fix.

Swapped out models don't have a _default_manager unless they have
explicitly defined managers. ModelState.from_model() now accounts for
this case and uses an empty list for managers if no explicit managers
are defined and a model is swapped out.

If Django is installed in a path that contains non-ASCII characters,
the tag failed with UnicodeDecodeError.

Refs #24324 -- Fixed Python 2 test failures when path to Django source contains non-ASCII characters.

The message for the SuspiciousFileOperation exception needs to
be a unicode string.

The function implemented most of upath(), but skipped the check for
strings that are already unicode.

If the project path contained a non-ASCII character, Python 2 crashed.

If 'name' contained non-ASCII characters, the comparison raised a
UnicodeEncodeError on Python 2.

Without this migration, the auth signal handlers will fail if migrating
only auth.