Allow prefilling of key-values pairs with primitive values on investigation and study level via config

[patrick-koenig]

Examples:

window.config = {

    prefill: [
        {
            type: 'value',
            isaMapping: {
                entity: 'investigation',
                attribute: 'title'
            },
            value: 'Prefilled investigation title ...'
        },
        {
            type: 'value',
            isaMapping: {
                entity: 'study',
                attribute: 'description',
                studyIndex: 0
            },
            value: 'Prefilled study description ...'
        }
    ]
}