Use frontend host as backend host per default

[mmabrouk]

The frontend calls the backend using process.env.NEXT_PUBLIC_AGENTA_API_URL

For instnace

export const createAppFromTemplate = async (
    templateObj: AppTemplate,
    ignoreAxiosError: boolean = false,
) => {
    const response = await axios.post(
        `${process.env.NEXT_PUBLIC_AGENTA_API_URL}/api/apps/app_and_variant_from_template/`,
        templateObj,
        {_ignoreError: ignoreAxiosError} as any,
    )
    return response
}

We want to call the same host when NEXT_PUBLIC_AGENTA_API_URL is empty.

One solution is to use ${window.location.host} when NEXT_PUBLIC_AGENTA_API_URL is set

This is needed to fix another issue in deployment

[bekossy]

Fixed in #970