Fix opening the menu in a Share page

".popovermenu" elements are visible or not depending on whether they
also have the "open" CSS class or not. "#header .menu" elements were
always hidden, so when both rules applied to the same element, like in
the menu of a Share page, the element was always hidden due to
"#header .menu" being more specific than ".popovermenu" and thus
overriding its rules. Now, "#header .menu" elements are hidden only if
they are not a ".popovermenu" too.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>