Lead with "GitHub" instead of "Git" for clarity of the focus of our Training services

[matthewmccullough]

Change all the course (not class notes) advertisements to consistently be "GitHub and Git" or just "GitHub" as appropriate. Legacy "Git and GitHub" phrasing can be found with this search.

[jordanmccullough]

Updated teach.github.com via this PR.

[matthewmccullough]

@github/training-team Are Teach.github.com and Training.github.com the two properties we need to concern ourselves with for this wording update? Any others to add here? If no, let's close this and call it double 🚢ed!

[brntbeer]

Call it shipped.

[jroll-zz]

Thanks so much for doing this @matthewmccullough! I had it on my list but hadn't gotten to it yet :(

[matthewmccullough]

We are all here to help each other! I'm always happy to help my wonderful colleagues. ❤️