Provide dynamic fields mixin for serializers

Description / Overview

Dynamic fields, as in /submission/3/?fields=text will only return the text and nothing else.

Use cases

Provide a DRY way to enable the front-end to get only the data that is needed.

Links / references

https://github.com/dbrgn/drf-dynamic-fields http://www.django-rest-framework.org/api-guide/serializers/#dynamically-modifying-fields

Feature checklist

added Assigned and removed Needs assignment labels

added review and removed Assigned labels

assigned to @robinwilliam.hundt

Closed by 2cc91192.

closed