refactor(GroupSerializer): serialize group permissions and dataset slug

[goldpbear]

This PR serializes permissions for each dataset group returned along with the User serialization. Group permissions are serialized as an array in the following format:

[
  {
    abilities: ["create", "retrieve", "update", "destroy"],
    submission_set: "*"
  }
]

where each object in the array represents one set of permissions.

We also serialize the dataset_slug of each group's parent dataset.