Record SPDX license identifier in the License model

[Tom Pollard]

As discussed in #2145, it would be nice to add license information to the DataCite metadata profile for projects. DataCite encourages use of SPDX identifiers (an identifier scheme for licenses: https://spdx.org/licenses/) when doing this:

      "rightsList": [
        {
          "rights": "Creative Commons Zero v1.0 Universal",
          "rightsUri": "https://creativecommons.org/publicdomain/zero/1.0/legalcode",
          "schemeUri": "https://spdx.org/licenses/",
          "rightsIdentifier": "cc0-1.0",
          "rightsIdentifierScheme": "SPDX"
        }
      ],

We should add a field to the License model to associate licenses with their SPDX identifier (https://spdx.org/licenses/). Note that some licenses will not have an identifier, so the field should be optional.

See also: https://en.wikipedia.org/wiki/Software_Package_Data_Exchange