Migrate from react-scripts to gatsby

This is based on #17 which should be merged first

Replaces react-scripts with gatsby.

Gatsby has a few benefits.

simple MDX support allowing for east content update
Ability to pre-render certain content that rarely changes while stiff doing live client-side queries
a wide ecosystem of plugins and data sources allowing for the simple addition of offline support and such
allows of simple config to control data such as social info and menus.

redbrick / react-site