FEATURE: New site setting min_flags_staff_visibility

When set higher than 1, flags won't show up for staff in the admin
section unless the minimum threshold of flags on a post is reached.

app/models/post_action.rb

@@ -52,13 +52,22 @@ class PostAction < ActiveRecord::Base
   end
 
   def self.update_flagged_posts_count
-    posts_flagged_count = PostAction.active
+    flagged_relation = PostAction.active
       .flags
       .joins(post: :topic)
       .where('posts.deleted_at' => nil)
       .where('topics.deleted_at' => nil)
       .where('posts.user_id > 0')
-      .count('DISTINCT posts.id')
+      .group("posts.id")
+
+    if SiteSetting.min_flags_staff_visibility > 1
+      flagged_relation = flagged_relation
+        .having("count(*) >= ?", SiteSetting.min_flags_staff_visibility)
+    end
+
+    posts_flagged_count = flagged_relation
+      .pluck("posts.id")
+      .count
 
     $redis.set('posts_flagged_count', posts_flagged_count)
     user_ids = User.staff.pluck(:id)