"The dissemination of the individual's opinions on matters of public interest is for us, in the historic words of the Declaration of Independence, an 'unalienable right' that 'governments are instituted among men to secure.' History shows us that the Founders were not always convinced that unlimited discussion of public issues would be 'for the benefit of all of us' but that they firmly adhered to the proposition that the 'true liberty of the press' permitted 'every man to publish his opinion'."