Fix Enrollment Email UI/UX

@jrbl 

Fixed enrollment email UI in Studio. The list of changes is as follows:
    - Test Email functionality: can now send yourself a test email from studio
    - Set default template button: can now click a button to replace email
        with the default template
    - UI changes: html ordering and structuring
                  Got rid of enable_default_email button. Pushed this feature
                  to the default template button

Things to note:
    - default template is stanford-specific.
    - minor test changes (since enable_default_email field is gone now)