Bulk-email now have links in respective to settings.HTTPS

Bulk emails always have links on https version of course, dashboard and course image.

Well, this PR fixed it. Easy-cheezey.

I understand, there are no tests here. But I just cannot figure out, how test this one. If someone could help me to write tests for that...
`lms/djangoapps/instructor/enrollment.py:229` and `lms/djangoapps/instructor/views/legacy.py:1470` use `protocol` as variable name and I prefer this