Expand Text-format CiviMail window

By auto-collapsing Plain Text, this step is often overlooked by users, and it is best practice to include plain text for versions of emails for the 15%+ recipients who prefer reading or require plain text.

Suggestion on this page: http://drupal.demo.civicrm.org/civicrm/mailing/send to open up the Plain Text textarea by default. Creating a plain text version is advisable for all senders (and not all receiptients allow HTML) and thus we should encourage best practice our users.