Add `Dataset.donors` and `Publication.donors`

[shirey]

Many Datasets and Publications are now associated with multiple donors, but we only expose a single donor in the Dataset/Publication field donor in the indices.

Add a new field, Dataset.donors and Publication.donors, to the entity indices that contains a list of all donors that are associated with the Dataset/Publication, each with the same fields that are stored under the current Dataset.donor fields. If there is only one Donor associated with the dataset still list it as a single entry in the list.