Improve date format in "WordPress 15 years"...

[tobifjellner]

Location: https://meta.trac.wordpress.org/browser/sites/trunk/wp15.wordpress.net/public_html/content/themes/twentyseventeen-wp15/template-parts/header/site-branding.php?marks=26#L26

$date_format = _x( 'F dS, Y', 'Site description date format' );
Would better be
$date_format = _x( 'F jS, Y', 'Site description date format' );

The current format would give "January 01st, 2018".
The suggested string would give "January 1st, 2018".
If the date that will be presented is day 10 or later in the month, nobody would notice this, but still better to use the correct formatting, right?

[iandunn]

In 6931:

WP15: Change to unpadded day format for cleaner presentation.

Props tobifjellner
Fixes #3529