Improve/Fix sign in flow

Description

Currently the sign in flow just gets the token and put it on the localStorage, check and implement the best approach on Nuxt, adapt the menu and add the logout action as well.

Definition of Done

• After signing in, you see Logout on the menu instead of Sign in;
• The routes under auth folder on pages are protected from unauthenticated users;
• The model and dataset menus are shown only after sign in;