Web API: move content checksums under a common JSON key

Each checksum currently appear as a separate top-level key in content medata (see blake2, sha1, sha256 in this example).

We should move all checksums under a common top-level key like "checksums", for better organization and introspection, e.g.:

{
    "checksums": {
        "blake2s256": "6fa16ac015c6513f6b98ee9e3f771ca8324a0ce77fbb9337fe3f8f549643dc73",
        "sha1": "adc83b19e793491b1c6ea0fd8b46cd9f32e592fc",
        "sha1_git": "8b137891791fe96927ad78e64b0aad7bded08bdc",
        "sha256": "01ba4719c80b6fe911b091a7c05124b64eeece964e09c058ef8f9805daca546b"
    }
    "data_url": "/api/1/content/sha1:adc83b19e793491b1c6ea0fd8b46cd9f32e592fc/raw/",
    "filetype_url": "/api/1/content/sha1:adc83b19e793491b1c6ea0fd8b46cd9f32e592fc/filetype/",
    "language_url": "/api/1/content/sha1:adc83b19e793491b1c6ea0fd8b46cd9f32e592fc/language/",
    "length": 1,
    "license_url": "/api/1/content/sha1:adc83b19e793491b1c6ea0fd8b46cd9f32e592fc/license/",
    "status": "visible"
}

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Migrated from T779 (view on Phabricator)