Refactor the main page
This commit makes users mobile friendly. To not have to maintain two menus, the menu was exported to a _menu.html.erb and is included as part of a navbar that is only visible on small devices and contains a hamburger dropdown menu. The menu is also as a side menu for devices that have bigger screens. The main navigation items (username and logout) as well as the language menu were integrated in the menu, so that we don't have to maintain 3 menus (which we would have to somehow display on mobile). The 'card' container was replaced, because with the elements in the card-title integrated in the menu and without a border it did not make sense anymore.