Add callbacks to vulcan:routing SSR hooks
Add callbacks to vulcan:routing SSR hooks

This method allows for access to the SSR app component and associated css in the routing's hook functions without needing to hack core files.

This makes it possible, for example, to add MUI wrapper components and inject compiled css with the JSS library (see https://material-ui-1dab0.firebaseapp.com/guides/server-rendering).

Here's how I'd implement the callbacks in my own custom module: https://gist.github.com/wayfarerboy/96c0f1d13b3b7d3d2743501a1c7c0187
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