[8.x] Add when() and unless() methods to MailMessage

[KKSzymanowski]

This PR adds when() and unless() methods to the MailMessage class. The implementation is identical to the Illuminate\Database\Concerns\BuildsQueries::when() and Illuminate\Database\Concerns\BuildsQueries::unless() equivalents.

An example use case:

public function toMail(User $user)
{
    return (new MailMessage)
        ->when(
            $this->offer->acceptation_comment,
            function(MailMessage $message, $comment) {
                $message->line('Your offer has been rejected with a comment:')
                        ->line($comment);
            },
            function(MailMessage $message) {
                $message->line('Your offer has been rejected.');
            }
        );
}

During implementation I noticed I could just copy the methods from BuildsQueries trait without any changes, similar with the tests. I think this may be a good time to refactor these methods out of the BuildsQueries trait to something more generic and utilize them in other places without code duplication. I cannot think of any good place for them, though. What do you think?

Removing them from BuildsQueries and putting them somewhere else could however break backwards compatibility if someone uses this trait directly but that would need further testing to confirm.