search: add status:modqueue, status:unmoderated metatags.

* status:modqueue = ~status:pending ~status:flagged
* status:unmoderated = status:modqueue -user:self -approver:self -disapproval:any

app/models/post.rb

@@ -1645,7 +1645,9 @@ class Post < ApplicationRecord
       where("uploader_id = ?", user_id)
     end
 
-    def available_for_moderation(hidden, user = CurrentUser.user)
+    def available_for_moderation(hidden = false, user = CurrentUser.user)
+      return none if user.is_anonymous?
+
       approved_posts = user.post_approvals.select(:post_id)
       disapproved_posts = user.post_disapprovals.select(:post_id)