Renamed _posts with correct starting date in filename; Flex-box reorder ...

 @github/training-team This resolves the order-naming-hack of the Web/Git-Foundations events. With team approval, we can rename the /Advanced-Git/_posts as well.

Using flex-box ordering to reverse the column appearance. Support in WebKit, Moz, and IE10.

@cobyism, will you give this a review?