Fixed #23556 -- Raised a more meaningful error message when migrations refer to an unavailable node
Merge pull request #3280 from Markush2010/ticket23556

Fixed #23556 -- Raise a more meaningful error message when migrations refer to an unavailable node
Fixed #18731 -- Added an example about customizing "makemessages" command.

Thanks claudp for the suggestion and review.
Fixed #23725 -- Substituted AUTH_USER_MODEL for User in docs.
Fixed #23575 -- Added a code example for custom AdminSite.
Added missing imports to example from previous commit.
Fixed #23558 -- documented slugify limitations
Updated FIRST_DAY_OF_WEEK for Ukrainian to Monday.
Fixed #8149 -- Made File.__iter__() support universal newlines.

The following are recognized as ending a line: the Unix end-of-line
convention '\n', the Windows convention '\r\n', and the old
Macintosh convention '\r'.

http://www.python.org/dev/peps/pep-0278

Thanks tchaumeny for review.
Fixed #23152 -- Added support for transactional Spatialite metadata initialization.

Thanks Doug Goldstein for the initial patch.
Clarified BaseCommand --traceback behavior.
Fixed #23733 -- Fixed squashing migrations that depend on multiple apps.
Converted seealso directives to use 4 space indendation.
Removed extra period.
Explained why admindocs omits model methods with arguments.
Fixed #23734 -- Replaced striptags in template filter overview since it has security implications.

Thanks Aymeric Augustin for the suggestion.
Fixed #23737 -- Recommended the render() shortcut more strongly.

Thanks Aymeric Augustin for the report.
Fixed #23736 -- Corrected the description of the value that silent_variable_failure uses.

Thanks Aymeric Augustin for the report.
Fixed #23735 -- Clarified that admindocs must be enabled to view template tag libraries in admin.

Thanks Aymeric Augustin for the report.
Fixed #23656 -- Made FormMixin.get_form's form_class argument optional.

Thanks Tim Graham for the review.