Use Doctocat Header component

[shawnbot]

I had a hell of a time getting the header from Blueprints working in the new Gatsby set up, and documented @colebemis's suggestion to use Yarn's selective version resolution feature to work around mismatched React versions. @emplums requested that we switch back to npm for consistency with other projects, and I agreed that we should do this as soon as we're able to swap out the Blueprints header for the one in Doctocat.

[colebemis]

Closed by #170