SEO Tools: support taxonomy archive pages in page titles. (#12425)

The SEO Tools allows you to set a custom title for different types of pages, from your WordPress.com dashboard.
However, once you've done that and when viewing a taxonomy archive page, you get an incomplete title. This fixes is, since taxonomy archive pages are very similar to non-custom taxonomy archive pages like category or tag pages.

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

@@ -156,7 +156,7 @@ public static function get_page_type() {
 			return 'front_page';
 		}
 
-		if ( is_category() || is_tag() ) {
+		if ( is_category() || is_tag() || is_tax() ) {
 			return 'groups';
 		}