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.
Fixed #19908 -- Added missing import in forms.py

Thanks loic84 for the report.
Changed testing strategy used in 6b03179e.

Avoid polluting the app cache as it causes unrelated test failures.

Refs #19688.
Minor edits to some recently-added admin docs.
Remove leading underscore from a function that's all growed up now.

This function is now the de facto standard function for rendering values in a
template, and is imported by two other built-in template modules. It shouldn't
have a leading underscore.
Switching to autoescape is not a change in syntax.