Collapse site content into training.github.com

[matthewmccullough]

Like @jordanmccullough's effort to fold teach into training for easier discovery of learning content in one unified site, we should also collapse learn.github.com into training.github.com through a merge of as much content as we can to preserve authorship info, and then a 301 redirect to the training.github.com domain of an equivalent URL.

[jordanmccullough]

The PR for this on training.github.com has been opened here. Closing this now in favor of pending PR approval.