rest-api: expose "/api/identifiers/<identifier>" endpoint

[slint]

An endpoint which exposes the identity + metadata information for an identifier would help a lot for inspection but also resolving alternate identifiers. This should serve "live" results from the database instead of ES. Example:

GET /api/identifiers/2018zndo...1226458A

{
    "ID": "92e3bb48-bccd-42bf-8057-276e1227ace0",
    "Created": "2019-01-01T12:00:00Z",
    "Updated": "2019-06-15T15:30:00Z",
    "Title": "Trackpy: Trackpy V0.4.1",
    "Type": {"Name": "software"},
    "PublicationDate": "2018-04-21",
    "Publisher": [{"Name": "Zenodo"}],
    "Creator": [
        {"Name": "Allan, Daniel B."},
        {"Name": "Caswell, Thomas"},
        {"Name": "Keim, Nathan C."},
        {"Name": "van der Wel, Casper M."}
    ],
    "Identifier": [
        {
            "ID": "10.5281/zenodo.1226458",
            "IDScheme": "doi",
            "IDURL": "http://doi.org/10.5281/zenodo.1226458"
        },
        {
            "ID": "2018zndo...1226458A",
            "IDScheme": "ads",
            "IDURL": "http://ui.adsabs.harvard.edu/#abs/2018zndo...1226458A"
        },
        {
            "ID": "https://zenodo.org/record/1226458",
            "IDScheme": "url",
            "IDURL": "https://zenodo.org/record/1226458"
        },
        {
            "ID": "https://github.com/soft-matter/trackpy/tree/v0.4.1",
            "IDScheme": "url",
            "IDURL": "https://github.com/soft-matter/trackpy/tree/v0.4.1"
        },
    ],
}