Add display name to pizza order API

Gijs Hendriksenrequested to merge
add-display-name-to-pizza-order-api into master

Necessary for #89 (closed)

Short description

Adds a new field, display_name, to the pizza orders API, which contains either the member's name, or the name entered by the organising committee. This way, all names can be neatly displayed in an admin screen in the app.

Merge request reports