Stopped stripping microseconds with MySQL backend

Refs #19716.
Fixed #19716 -- Added support for microseconds with MySQL 5.6.4 and up

Thanks erik@cederstrand.dk for the report and Tim Graham for the review.
Required MySQLdb >= 1.2.5 for fractional seconds support

On MySQLdb < 1.2.5, MySQLdb returns None when fetching datetime/time
values with fractional seconds.
See https://github.com/farcepest/MySQLdb1/issues/24
Fixed #23606 -- Implemented Client and RequestFactory trace() methods.

Thanks KevinEtienne for the suggestion.
Fixed flake8 warnings.
Fixed spelling errors in docs.
Limited lines to 119 characters in django/{contrib,db}.

Refs #23395.
Fixed #23719 -- Fixed MySQL 5.6 crash with GeometryFields in migrations.
Forwardported 1.7.2 release note from a0cfd77971dc0e61427ca468e9b6ea9a548c796e.
Fixed #23614 -- Changed the way the migration autodetector orders unique/index_together

Thanks to Naddiseo for the report and Tim Graham for the review
Added cross reference in admin docs to topic about saving objects in formsets.
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.