accesslib: Introducing context_moved() - call after moving courses/coursecats

accesslib: Introducing context_moved() - call after moving courses/coursecats

With the new accesslib, moving courses and categories has a major
impact on enrolments and unenrolments.

At _least_ we need to signal accesslib that it has happened. So here
is context_moved() for exactly that.

Open to refactoring later into something along the lines of

 - move_course()
 - move_category()

However, at this stage the most important of those two: move_course()
does not fit very well with the code in course/edit. So keep it simple
for now.