Flush rewrite rules upon theme switch

Themes can ( and do ) but shouldn't include custom rewrite rules.This can lead to hard to debug issues for theme authors. Theme changes are not a ultra common conclusion. Flushing the rewrite rules on theme switch will lead to a clean slate for each theme which helps make debugging easier.

And @nacin said we should do this 5 years ago at WordCamp Mid Atlantic.

Fixes #14849


Built from https://develop.svn.wordpress.org/trunk@34028


git-svn-id: http://core.svn.wordpress.org/trunk@33997 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/theme.php

@@ -1888,6 +1888,7 @@ function check_theme_switched() {
 			/** This action is documented in wp-includes/theme.php */
 			do_action( 'after_switch_theme', $stylesheet );
 		}
+		flush_rewrite_rules();
 
 		update_option( 'theme_switched', false );
 	}

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.4-alpha-34027';
+$wp_version = '4.4-alpha-34028';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.