require login to see course info, static tabs, and syllabus

Currently, some static tabs are visible to non-enrolled students. Other tabs (like course info) may appear, but be stripped of content. This makes it seem as though the platform is breaking.

This change makes courseware tabs visible only to logged in users.

Addresses: https://edx-wiki.atlassian.net/browse/LMS-507

@cpennington 
@nedbat