Add support for RightsStatements.org to entity data model

[GeoffFroh]

The DDR needs to support the rightsstatements.org vocabulary (https://rightsstatements.org/en/). The Entity model needs to have a rights_statement attribute that can be populated with a single id value from the controlled vocabulary in densho-vocabs:/api/0.2/rightsstatements.json.

There is an attribute called rights_statement in the existing model. Its purpose is to store a text description of additional information about access rights and restrictions. Thus far, it has not been used very often, however, any existing data should be moved to a new attribute, rights_statement_additional. This data field is not currently used by the ddr-public app.

See the attached sample json entity file for proposed format.

ddr-cmdln-183-sample_entity.json.txt