User account tokens in _user_mail_text() should not be sanitized (leads to HTML entities appearing in email message subjects)

Steps to reproduce:

1. Create a Drupal 7 site with the name "Somebody's site".
2. As an administrator, create a new user account and check the box to send an e-mail notifying them of the account.
3. Read your e-mail. The message subject in your inbox will say An administrator created an account for you at Somebody's site.

The token substitution code in _user_mail_text() results in the token replacements being sanitized, so that check_plain() runs on them. As far as I know, this shouldn't be necessary at all; in the case of the message subject it results in the double-encoding shown above. In the case of the e-mail body, the default mail implementation runs http://api.drupal.org/api/function/drupal_html_to_text/7 on that before sending it, so we actually want the HTML tags to remain in there as is.