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>

core/css/header.scss

@@ -78,11 +78,14 @@
 		background-color: $color-main-background;
 		filter: drop-shadow(0 1px 3px $color-box-shadow);
 		border-radius: 0 0 3px 3px;
-		display: none;
 		box-sizing: border-box;
 		z-index: 2000;
 		position: absolute;
 
+		&:not(.popovermenu) {
+			display: none;
+		}
+
 		/* Dropdown arrow */
 		&:after {
 			border: 10px solid transparent;