Delete User data when a user is deleted

[thomasplevy]

Migrated from #263

• [ ] Unschedule recurring payments, delete orders
• [ ] Delete data from lifterlms_user_postmeta table
• [ ] Delete quiz attempts
• [ ] Delete events from the lifterlms_events table
• [ ] Delete notifications from the lifterlms_notifications table

[eri-trabiccolo]

I'm possibly going to break this in several PR so that the review process would be easier.

[thomasplevy]

Sounds good