Post.pending_or_flagged: fix ambiguous column reference.

Fixes an ambiguous column reference error when searching for
`status:unmoderated` on the comments page:

    https://danbooru.donmai.us/comments?group_by=comment&search[post_tags_match]=status:unmoderated

Also removes the `id in (...)` subquery checking whether flagged posts
have a flag from the last week; this is always true.

app/models/post.rb

@@ -1618,15 +1618,15 @@ class Post < ApplicationRecord
     end
 
     def pending
-      where("is_pending = ?", true)
+      where(is_pending: true)
     end
 
     def flagged
-      where("is_flagged = ?", true)
+      where(is_flagged: true)
     end
 
     def pending_or_flagged
-      where("(is_pending = ? or (is_flagged = ? and id in (select _.post_id from post_flags _ where _.created_at >= ?)))", true, true, 1.week.ago)
+      pending.or(flagged)
     end
 
     def undeleted