Update page shell

[katydecorah]

This PR will update the page shell. All other Assembly/React based sites are upgrading to Assembly v1. There shouldn't a visual change to the generated Jekyll page shell, however, we upgraded dependencies that generate the shell and the output has changed slightly.

We should check that the top navigation and footer display as expected in:

• [x] Chrome
• [x] Firefox
• [x] Safari
• [x] Edge
• [x] Mobile Safari
• [x] Android Chrome
• [x] IE11 (The browser support banners should appear)

Ref https://github.com/mapbox/documentation/issues/710

[katydecorah]

This branch is on staging.

[katydecorah]

This is ready for review