authentication links in the navigation bar and token verification

based on !28 (merged)

Solves #50 (closed)

Solves #51 (closed)

Add a user dropdown in the navigation bar that shows login and logout options.

If the login is successful, the received token is verified and the name of the user and whether an admin has logged in is written in the store.

If the user is not logged in, it shows: Login

If the user is already logged in, it shows: Profile and Logout

The Profile link is actually just a placeholder, it links nowhere, but should be used later to link to the user profile.