Extract default initializations into module

No PT story.

Extract `#set_defaults` method from Partner & Sponsor models into `#set_status` & `#set_start_date` methods within a new Initializer module.
Move this to the concern. To do this, in the concern you have to:

```
module Initializer
  extend ActiveSupport::Concern

  included do
    after_initialize :set_status, :set_start_date
  end

  # ... methods
end
```