Move site creation and migrations to RevertableStepProcessor

[kirtangajjar]

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.