Improve completion calculation in the Book module

Currently the completion calculation in the Book activity relies on the fact that we reach the last page of the book. This is probably not the best metric to see if we have "read"/viewed the book.

In order to improve this, I suggest we add a new completion table for this activity that would record for a given user which chapter have been viewed.

Once all chapters have been viewed, we can consider that this user has viewed the book.