Add alternative labels for input and links

See https://github.com/GoogleChrome/accessibility-developer-tools/wiki/Audit-Rules#ax_text_01

src/components/navigation/navigation.jsx

@@ -196,7 +196,7 @@ var Navigation = React.createClass({
         return (
             <div className={classes}>
                 <ul>
-                    <li className="logo"><a href="/"></a></li>
+                    <li className="logo"><a href="/" aria-label="Scratch"></a></li>
 
                     <li className="link create">
                         <a href="/projects/editor">
@@ -237,7 +237,10 @@ var Navigation = React.createClass({
                     <li className="search">
                         <form action="/search/google_results" method="get">
                             <Input type="submit" value="" />
-                            <Input type="text" placeholder={formatMessage(defaultMessages.search)} name="q" />
+                            <Input type="text"
+                                   aria-label={formatMessage(defaultMessages.search)}
+                                   placeholder={formatMessage(defaultMessages.search)}
+                                   name="q" />
                             <Input type="hidden" name="date" value="anytime" />
                             <Input type="hidden" name="sort_by" value="datetime_shared" />
                         </form>

src/views/about/about.jsx

@@ -26,6 +26,7 @@ var About = React.createClass({
 
                     <div>
                         <iframe
+                            title="Scratch Overview Video"
                             src="https://player.vimeo.com/video/65583694?title=0&byline=0&portrait=0"
                             frameBorder="0"
                             webkitAllowFullScreen