Controllers use refUser, refCourse etc methods from model package

(The methods were accidentally included in the previous commit, and
with a poor choice of name).

This centralises what kind of Ref we create (and it's been changed to a
LazyId).  It also makes the controllers a little more readable.
Set some default content for courses and markdown pages

This also fixes a bug where an unreadable content item would be created
as page one for every course.