Fixed #24093 -- Prevented MigrationWriter to write operation kwargs that are not explicitly deconstructed

This possibility was documented but not tested.

It had been broken during the multiple template engines refactor.

This is for consistency with Template.render.

It adds a little bit of knowledge about HTTP requests in
django.template.loader but I think consistency trumps purity.

It's the main entrypoint to the refactored template system.

Thanks Tim Graham and Markus Holtermann for the reviews.

Moved list constants instantiation into optimizer's __init__.

Thanks to Russell Keith-Magee for mentoring this Google Summer of
Code 2014 project and everyone else who helped with the patch!

This is the expected behavior, but given RequestContext's tortuous
implementation, a straightforward use of its API results in the
opposite.

This commits fixes a regression that must have happened at different
points in the multiple templates engine refactor for different features.

Since this is a private API introduced in Django 1.8, no documentation
is required.

This adds support for authentication data (`user:password`) in URLs,
IPv6 addresses, and unicode domains.

The test suite has been improved by adding test URLs from
http://mathiasbynens.be/demo/url-regex (with a few adjustments,
like allowing local and reserved IPs).

The previous URL validation regex failed this test suite on 13
occasions, the validator was updated based on
https://gist.github.com/dperini/729294.

...with the theoretical assumption that gettext may once reach a
two-digit number. Thanks Walter Doekes for noticing this potential
issue.

Thanks ajenhl Trac user for the report.

Thanks Markus Holtermann for spotting this.

Thanks codeitloadit for the report, living180 for investigations
and Tim Graham for the review.

Thanks Berker Peksag for the review.

Thanks Markus Holterman and Berker Peksag for review.

Thanks jdetaeye for the report, bhuztez and jdetaeye for the
initial patches, Tim Graham and Berker Peksag for the reviews.

refs 67325669

Thanks Markus Holtermann for review and feedback.