base: reference field fix

* Adds raw_reference subfield to data model to store a raw reference.

Signed-off-by: Lars Holm Nielsen <lars.holm.nielsen@cern.ch>
deposit: reference field addition

* Adds reference field to deposit form.

* Adds display of references in records.

* Adds preliminary export of references in DataCite.

Signed-off-by: Lars Holm Nielsen <lars.holm.nielsen@cern.ch>