Render json for board lists using serializers

At the current state board lists are serialized using as_json method in the model which is difficult
to maintain, specially when adding new fields that are methods.

We should start using serializers + entities for lists like we do for most of the models.

The following discussion from gitlab-ce!31069 should be addressed:

• @engwan started a discussion: (+1 comment)

  Our best practices doc says we should create the issue and link to it here