Only track required changes in GitlabSubscriptionHistory

Summary

Currently, we create a GitlabSubscriptionHistory record whenever any attribute is changed on the GitlabSubscription.

But, we only track certain attributes on that model, so we risk potentially creating a bunch of GitlabSubscriptionHistory records unnecessarily, e.g. if last_seat_refreshed_at is updated, we'd create a new history record which doesn't show any difference to the current GitlabSubscription record.

Proposal

We can change the logic in GitlabSubscription to check if any changes have been made to tracked attributes, and only create a history record accordingly