Restructure navigation

[jgonggrijp]

Once #323 and #163 are implemented and we have a permanent solution for copyrighted sources (see #86), the navigations structure should change a bit:

• The need to authenticate should be lifted from the search and exploration screens and routes. After all, the interface is meant to engage the general public.
• The redirection from the / route should become conditional on whether the user is an anonymous visitor or an authenticated account holder. In the first, case, it should continue to redirect to the search screen, while in the second case, it should redirect to the personal landing page.