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.