Extract blocking layer in edge-boilerplate to a reusable component

[swernerx]

We currently block the interactions of the user in edge-boilerplate during the initialization time where React has not yet rehydrated the server rendered markup. This is currently somehow built into the boilerplate but should be better separated.

There are also some minor issues to think about:

• When JavaScript is disabled in the browser it might be useful to prevent the layer e.g. via some styles from a noscript tag. A server rendered app might still be - at least partly - usable without any JS.