User Onboarding: Calculate `percentageComplated` instead of using a hardcoded value

The percentageCompleted value is currently a hard coded value for each tour step in https://gitlab.com/gitlab-org/gitlab-ee/blob/master/ee/app/assets/javascripts/onboarding/tour_parts.js

This value should be calculated based on the lastStepIndex and the total number of steps for a particular tour.