remove 'preview' from urls in enrollment emails sent while previewing a ...

...course

A temporary fix to make sure, if an instructor sends out enrollment or unenrollment emails when previewing a course, that students do not receive emails to register at preview.<site_name>, but just at <site_name>. So instead of preview.edge.edx.org, a student would be directed to edge.edx.org. It's hacky, but temporary.

Addresses https://edx-wiki.atlassian.net/browse/LMS-726

@ormsbee 
@dianakhuang