Update images schema to use images-id instead of raw string

[etinquis]

{
    "$id": "https://data.landsofhope.com/schemas/images.json",
    "type": "object",
    "properties": {
        "M": {
            "type": "array",
            "items": {
                "$ref": "schemas/images-id.json"
            }
        },
        "F": {
            "type": "array",
            "items": {
                "$ref": "schemas/images-id.json"
            }
        },
        "U": {
            "type": "array",
            "items": {
                "$ref": "schemas/images-id.json"
            }
        }
    },
    "additionalProperties": false,
    "anyOf": [
        {
            "required": [
                "M"
            ]
        },
        {
            "required": [
                "F"
            ]
        },
        {
            "required": [
                "U"
            ]
        }
    ]
}