AWS settings hook for addl installed apps + lms/wsgi_apache.py

wsgi_apache.py passes an apache env variable for SERVICE_VARIANT
to the os environment, where it's normally set when we use gunicorn
Initial commit for enabling shibboleth logins using external_auth
create external_auth migrations
remove latent testing code
hardcode service variant into wsgi_apache_lms.py
enable login via shib and also enrollment restriction
remove debug print
switch login and register buttons to use when the course is restricted
Addresses several review comments: var name change and dispatcher

* changes CMS advanced setting variable name to enrollment_domain
* adds central dispatcher for course specific reg. and login
* wraps error message in _ and dedent
Use mock POST request for change_enrollment after shib login (GET)
ask user for email during registration if it's not given by shib