Use HTML::Proofer

Per https://github.com/github/training.github.com/issues/129#issuecomment-30165276.

The (new) tests are failing... does anyone feel strongly about me fixing them in this PR, vs. a separate one?  *Technically* `master` is "broken" by these new standards, but is being incorrectly reported as "passing".  P.S. this site has a *lot* of content... the `cibuild` took nearly 10 minutes on my local machine :flushed: 

Was gonna paste the output here, but there are "1367 failures" :grimacing:  Put it in [a Gist](https://gist.github.com/afeld/da193daa4e871e4a5c41) instead.