Fix double 'and' in repostedBy and empty names edge case
formatRepostedBy was pushing an 'and X other people' suffix into
the names array then passing it to oxfordComma(), which added a
second 'and' before the last item. Also handled the case where
the repostedBy array is empty but repostCount is non-zero (API
returns up to 25 but the count can be higher).