Only show active partners in API

Ghost User
2017-10-03T15:46:23+02:00

Closes no issue

#### Previous behaviour
Steps to reproduce:

1. Create an inactive partner.
2. Go to `/api/v1/partners/`.
3. Notice that the inactive partner is included in the list and that there is no way to tell that the partner is inactive.

#### New behaviour
Steps to validate that it works:

1. Create an inactive partner.
2. Go to `/api/v1/partners/`.
3. Notice that the inactive partner is no longer included in the list.