Fix users list dropdown position after scroll

When scrolling, the position calculation of the multiselect dropdown was
wrong. Adding "position: relative" to the list container makes the
button calculation relative to it instead of the whole page. In this
case the dropdown is properly aligned with the field regardless of
scrolling.

settings/css/settings.css

@@ -171,6 +171,11 @@ td.password>img,td.displayName>img, td.remove>a, td.quota>img { visibility:hidde
 td.password, td.quota, td.displayName { width:12em; cursor:pointer; }
 td.password>span, td.quota>span, rd.displayName>span { margin-right: 1.2em; color: #C7C7C7; }
 span.usersLastLoginTooltip { white-space: nowrap; }
+
+/* dropdowns will be relative to this element */
+#userlist {
+	position: relative;
+}
 #userlist .mailAddress,
 #userlist .storageLocation,
 #userlist .userBackend,