Initial optimisations and bugfixes

Initial optimisations and bugfixes:

Upgrade Babel dependencies from 7.4.x to 7.5.x #486
Add missing lang attribute from shell component document element #487
Bugfix; Add x-overflow: hidden to shell component styles #488
Bugfix; sticky header should always have the --simple style variant #489
Adds a webpack bail rule to dotcom-page-kit-cli #491