FEATURE: change default size for search recent posts

The site setting `search_recent_posts_size` controls the window of posts
that we will search through before trying to search through the full index

If this number is too low then the search quality can suffer a lot as recent
posts may dominate search. If the number is too high then performance will
suffer.

This attempts to find a happy medium, 1 million posts will cover the majority
of forums out there and should perform adequately.

config/site_settings.yml

@@ -1845,7 +1845,7 @@ search:
   search_tokenize_chinese_japanese_korean: false
   search_prefer_recent_posts: false
   search_recent_posts_size:
-    default: 100000
+    default: 1000000
     max: 10000000
   search_recent_regular_posts_offset_post_id:
     default: 0