fix shib reg from course about page

@sarina  @dcadams 
It seems weird to me that this is parameterized separately in two files and is now being used in a third - it might be time to make `SHIBBOLETH_DOMAIN_PREFIX` into an env variable? https://github.com/edx/edx-platform/blob/master/lms/djangoapps/instructor/enrollment.py#L22 and https://github.com/edx/edx-platform/blob/master/common/djangoapps/external_auth/views.py#L56

Also I'm wondering if it would be cleaner to be determining `is_shib_course` in the views and passing along the value through to this template.