SEO: do not display any tags in post titles display in meta tags

Fixes 4478-gh-jpop-issues

Some plugins use the the_title filter to add HTML to post titles (to add schema.org metadata for example).
We consequently cannot assume anything returned by get_the_title() will be safe to display in a meta tag.
Let's use the_title_attribute() instead to avoid having any unwanted HTML.

modules/seo-tools/jetpack-seo-titles.php

@@ -131,7 +131,7 @@ public static function get_token_value( $token_name ) {
 
 			case 'post_title':
 			case 'page_title':
-				return get_the_title();
+				return the_title_attribute( array( 'echo' => false ) );
 
 			case 'group_title':
 				return single_tag_title( '', false );