minor refactoring: set up facebook test manually instead of subclassing FacebookPageTest

...since subclassing duplicates the tests in both classes, so running the tests takes longer.
minor test refactoring: reuse self.activity setup
PPD: don't store blank SyndicatedPosts if we didn't fetch the author's site

@kylewm, mind sanity checking my logic here? the intuition seems pretty clear - if we don't fetch and process the h-feed (e.g. in handlers), we don't actually know whether there's a syndication relationship or not, so we shouldn't store a blank.