The `posts` relation on `Topic` is not ordered. Using `Topic.posts.first`
is basically the same as asking for a random post, it will depend on DB
order. This breaks on Topic merge and split for example.
Additionally, a huge problem with that is that it forces active record down
a slow path. `Topic.posts.first` is extremely slow on giant topics, since
it has no default ordering it appears AR materializes the entire set prior
to doing `first`.
This commit also illustrates the importance of testing, initially I only
fixed the second instance of the problem in `post_validator.rb` but testing
revealed that the problem was repeated at the top of the file.
Longer term we should consider a larger change of default ordering the posts
relations so people do not fall down this trap anymore.
