Reduce width of the admin menu on large screens

On very large screens, the admin menu had a lot of blank space for
languages where all sections had short names (like English). This was
inconvenient because the icon to open a submenu was far from its
associated menu item.

Using the `max-content` value for the `max-width` property, we reduce
the amount of blank space in these cases.

app/assets/stylesheets/admin/menu.scss

@@ -176,6 +176,7 @@
 
     a {
       font-weight: normal;
+      margin-#{$global-right}: 0;
     }
 
     .is-active {

app/assets/stylesheets/layout.scss

@@ -460,6 +460,7 @@ button,
     > nav {
       flex: 20%;
       min-width: $side-menu-min-width;
+      max-width: max-content;
 
       + * {
         flex: 80%;