Let instructors and volunteers assign articles

For instructors and volunteers, displays "Add an article" controls for
all students in their courses, on the course page. This allows said roles
to assign articles to students. See bug 56003. Also fixes issues in the
layout of the article table on course pages.

Bug: 56003
Change-Id: Ib9d036c4760b4ba847813e7cd4430bf4432109bc
Merge "Let instructors and volunteers assign articles"