[1/11] Page template headers

[philipnrmn]

This PR introduces the new Page Header components. In order to keep noise low, I've stayed relatively faithful to the original interface of the components.

The page header is wrapped in a utility component and added to the Page namespace in order to allow subclasses of Page to override it.

The intent is to allow every view to control its header by defining itself as a page, eg

<Page>
  <Page.Header breadcrumbs={...} />
  {usual page content here}
</Page>

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[orlandohohmeier]

Let's create issues for the open todos and get this one merge so that it no longer blocks #1483

[orlandohohmeier]

✨ Awesome

[mlunoe]

Let's wait with ternary until we have decided to start using it

[mlunoe]

done

[orlandohohmeier]

❤

[mlunoe]

Looks good! Let's ship