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



git-svn-id: https://develop.svn.wordpress.org/trunk@34028 602fd350-edb4-49c9-b593-d223f7449a82

src/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 );
 	}