Allow coaches access to teacher's guide materials in course tree

[pdpinch]

As a coach, I'd like to be able to access resources designed by the course author for coaches only, such as a Teacher's Guide. This should be easily accessible by coaches, but not by students.

One approach would be to allow coaches to view blocks that a marked hidden_from_students. However, this may overload the meaning of that field.

Another approach would be to introduce a block specifically for this purpose and only display it in a context that the coach has access to (i.e. the master course, or the coach's dashboard)

This may be a duplicate of #168