SAN FRANCISCO, Oct 20 (Reuters) - The United States can designate foreign organizations as terrorist groups and bar Americans from financially backing them, a federal appeals court ruled on Thursday. "Leaving the determination of whether a group is a 'foreign terrorist organization' to the executive branch ... is both a reasonable and a constitutional way to make such determinations," Judge Andrew Kleinfeld wrote for a three-judge panel. "The Constitution does not forbid Congress from requiring individuals, whether they agree with the executive branch determination or not, to refrain from furnishing material assistance to designated terrorist organizations." The ruling by the...