CiviMail: composing mailing body on-screen

Allow user to compose email & text messages on screen rather than upload files. We'll likely use the Dojo HTML Wysiwyg editor for this.

If there are existing active civicrm_msg_template records for the domain, then the user should also be given the option to select a template from a drop-down to prepopulate the "compose" block. If they modify the template contents, they should also be given the option to save those changes back to the template, OR save as a new template (OR neither)... when submitting the compose mailing form.