Merge pull request #717 from slurms/ticket_19746

Fixed #19746 -- Allow deserialization of pk-less data
Test case and docs for custom context data in feeds

Thanks Paul Winkler for the initial patch. (Ref #18112).
Added a note about Feed.get_context_data to the 1.6 release notes.
Fixed creation of html docs on python 3.

Thanks to Alan Lu for the report and the patch.
Fixed #15849 -- Made IfChanged node thread safe.

Previously, the ifchanged node stored state on `self._last_seen`,
thereby giving undesired results when the node is reused by another
thread at the same time (e.g. globally caching a Template object).

Thanks to akaihola for the report and Diederik van der Boor and
Bas Peschier for the patch.
Fixed #17320 -- Added whitespace validation to the Site.domain field
Fixed #19890 -- ifchanged templatetag rendered its content twice

The content of ifchanged template tag was rendered twice: first time, to
compare it with the previous value and the second time, to return the
rendered output.
Fixed #19394 --Added note about auth forms and custom user models.
Fix documentation :mod: role in Syndication feed text.
Merge pull request #832 from chrismedrela/ticket19890

Fixed #19890 -- ifchanged templatetag rendered its content twice
Fixed #19688 -- Allow model subclassing with a custom metaclass using six.with_metaclass
Merge pull request #828 from zerok/tickets/17320

Fixed #17320 -- Added whitespace validation to Site.domain field
Add column-<field_name> classes to the admin list

This simplifies CSS styling to set column widths.
Merge pull request #834 from vdboor/issue_19906_list_classes

Added column-<field_name> classes to the admin list
Fixed python 3 support. Refs #17320
Fixed some docstrings that were raising Sphinx warnings when running the admindocs tests.
Merge pull request #835 from vdboor/docfix

Fixed documentation typo (:mod: role in Syndication feed text)
Added missing versionadded for sqldropindexes command docs
Merge pull request #824 from ambv/languagecode
Fixed a test that was failing in Python 3.
The issue was that as of Python 3, the generators' `next()`
method becomes `__next()`. Thanks Alex Gaynor for noticing that.
Refs #19890.