Whitelist

Right now, we get `destroyable_associations` of an object by iterating through `reflect_on_all_associations`. Then we check if association has `dependent` option set. Either to `destroy` or `delete_all`. The main logic lives in: https://github.com/3scale/porta/blob/master/app/workers/delete_object_hierarchy_worker.rb

The new idea is to create a new option for an object/model that would says, if the object should be deleted on the background and by which method. Something like:

```
class MyNewModel < ActiveRecord::Base

self.background_delation = :destroy
```