Move site creation and migrations to RevertableStepProcessor

Currently site creation (both html and wp) uses its own creation and rollback logic. Instead of that we can use `RevertableStepProcessor` for it. This will make code clean and less messy than it is. Also it would give package developers a reference on how such actions with cleanup can be developed in EE.

Also, Migrations have very similar functionality. Hence it could be swapped out with `RevertableStepProcessor`.