MDL-23344 new option to autofocus username or password on the login form

Nearly everybody else does this login page autofocus. Once we switch to HMTL 5 we should use the new autofocus input option. I believe the admin setting should be enabled by default but it would not be possible to get it through the reviews.

lib/javascript-static.js

@@ -739,6 +739,34 @@ M.util.get_string = function(identifier, component, a) {
     return stringvalue;
 };
 
+/**
+ * Set focus on username or password field of the login form
+ */
+M.util.focus_login_form = function(Y) {
+    var username = Y.one('#username');
+    var password = Y.one('#password');
+
+    if (username == null || password == null) {
+        // something is wrong here
+        return;
+    }
+
+    var curElement = document.activeElement
+    if (curElement == 'undefined') {
+        // legacy browser - skip refocus protection
+    } else if (curElement.tagName == 'INPUT') {
+        // user was probably faster to focus something, do not mess with focus
+        return;
+    }
+
+    if (username.get('value') == '') {
+        username.focus();
+    } else {
+        password.focus();
+    }
+}
+
+
 //=== old legacy JS code, hopefully to be replaced soon by M.xx.yy and YUI3 code ===
 
 function checkall() {