"Individual rights have been recognized by our jurisprudence only after long and costly struggles. They should not be struck down by anything less than the gravest necessity. We assent to their temporary suspension only to the extent that they constitute a clear and present danger to the effective prosecution of the war and only as a means of preserving those rights undiminished for ourselves and future generations. Before giving such an assent, therefore, we should be convinced of the existence of a reasonable necessity and be satisfied that the suspension is in accordance with the legislative intention."