Fix course create page protection for non-faculty

For some reason, non-faculty were being incorrectly blocked from accessing
the group create page for non-courses. Not sure why this is just now
coming up; it looks like it's been this way since 2981eea6.

This changeset does two things:

- Ensures that the "Create a [group]" link goes to the right place, by
  making it an absolute rather than a relative link
- Ensures that the redirect away from the course creation page only
  happens when the current group type is indeed a course

See #936