Change: Show dedicated 404 page for invalid resource references

When visiting a public link or the `All files` page with an invalid resource in the URL (e.g. because it was deleted in the meantime) we now show a dedicated page which explains
that the resource could not be found and offers a link to go back to the respective root (»All files« home location or the root of the public link). The breadcrumbs have been made
available on invalid resources as well, so that those could be used for more precise navigation instead of jumping back to the root.

https://github.com/owncloud/web/pull/4411
