Bundled Themes: Make twentynineteen the default theme.

After [44149], we can now make twentynineteen the default theme. Twentyseventeen has been a great default, but 5.0 and twentynineteen is guten.

Merges [43809,43954] from the 5.0 branch to trunk.

Fixes #45152.
Props jorbin, SergeyBiryukov, pento, mcsf.



git-svn-id: https://develop.svn.wordpress.org/trunk@44151 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/class-wp-theme.php

@@ -53,6 +53,7 @@ final class WP_Theme implements ArrayAccess {
 		'twentyfifteen'   => 'Twenty Fifteen',
 		'twentysixteen'   => 'Twenty Sixteen',
 		'twentyseventeen' => 'Twenty Seventeen',
+		'twentynineteen'  => 'Twenty Nineteen',
 	);
 
 	/**