Fixed #18709 -- Check if initial_value is a callable

The fix was already in the patch for #18702, so only test added for
ticket #19895.

Slightly reworded 'last()' docs

In _get_changed_data, check if initial_value is a callable and call it
if it is.

Used formatted date instead of datetime object for the end of the
year range, as the datetime object loses fractions-of-seconds when
inserted into the db.

Removed the explicit casting of strings to dates

Report more details about template files in loader postmortem.

Fixed a Python 2.6 regression (GzipFile can't act as a context manager)

Use assertIsInstance in test code.

Gives much nicer errors when it fails.

Removed a confusing duplicate SESSION_COOKIE_DOMAIN header

Fixed #20472: response.content should be bytes on both Python 2 and 3

The note is clearly a part of MESSAGE_STORAGE documentation. As a separate
section, it broke automatic link generation on the HTML version of the
documentation.

This is backward incompatible for custom form field/widgets that rely
on the hard-coded 'Hold down "Control", or "Command" on a Mac, to select
more than one.' sentence.

Application that use standard model form fields and widgets aren't
affected but need to start handling these help texts by themselves
before Django 1.8.

For more details, see the related release notes and deprecation timeline
sections added with this commit.

Thanks Jeremy Dunck.

A regression caused by d5b93d32 made .get() error
reporting recurse infinitely on certain rare conditions. Fixed this by
not trying to print the given lookup kwargs.

When a GenericRelation was defined on abstract model, queries on childs
of the abstract model didn't work. The problem was in the way fields and
in particular field.rel was copied from models to their children.

The regression was likely caused by #19385. Thanks to Gavin Wahl for
spotting the regression.

Fix typo in redirect view docs

Changed API to disable ATOMIC_REQUESTS per view.