Found while looking at MDL-79191 - there is a 'grade' column in the completion entity

For the same reasons given in MDL-75640 when we implemented generic completion data for courses (rather that previously assuming only criteria of type "grade") we should do similar for completion data for users and not only assume criteria of type "grade"

Like MDL-76900, let's separate out all user grade data in to it's own entity so we get clearer and more in-depth reporting (a "grade" entity would be very useful in lots of other custom reports too)