Set preview text of campaign via API

[dompie]

I would like to set the preview text of a campaign via API. When I try it currently via the POST campaign endpoint, I get the following error:

Client error: `POST http://ops-keila-1:4000/api/v1/campaigns` resulted in a `400 Bad Request` response:  
{"errors":[{"pointer":"/data/preview_text","status":"400","title":"Unexpected field"}]}  

[wmnnd]

This was just fixed by @samuelc92 in #431 🚀