Adds is_xblock_visible_to_students (STUD-1814)

This method currently only checks release dates because staff lock doesn't exist yet.

@andy-armstrong @cahrens 
I was expecting this to be a method on a modulestore for some reason. Does this work for both old mongo and for split? Does it make sense to have one implementation for each, or can the helper method use higher level abstractions so that it doesn't need to care?