For modern Vector, removes the logout link in the user menu and places it
below that menu by appending it to the html-after-portal property of the
skin data.
Also modifies the `.vector-user-menu-login` style to accommodate both the
login and logout button.
bug: T281791
Depends-On: If82a736e37174aaadd0ff07019a1fae3759a9e51
Change-Id: I7675230e09a50eaeab448182329f850ad2689514
Jan Drewniak