refactor(app/navigation): split sidebar navigation, move diagnostics to configuration

[schogges]

This change splits up the sidebar navigation into a top nav including the nav links to the most important routes. The new bottom nav offers a place for other routes. Both navigation lists are separated by a visual separator. This ensures that the user finds all nav links to top level resources in the same area of the UI. Both nav lists are only split visually and remain within a single nav element. To further beautify the nav links I've added icons to each of them.

I've moved and renamed everything from diagnostics to configuration. This makes it more clear that this part of the UI shows the CP configuration.

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[johncowen]

P.S. I also just noticed there is something functionality wrong with the Configuration page, I think there's a test failing that could be related to that but currently getting this when searching when using the PR preview: