Add participant ID

Collect participant ID at the beginning, store in database, export with
results.  Closes https://github.com/mattbk/changeblindness/issues/2.

views/index.html.twig

@@ -4,7 +4,12 @@
     <header class="hero-unit">
         <h1>Change blindness test</h1>
         <p>We would like a few minutes of your time to help us with a test in change blindness. Your help is very much appreciated.</p>
-        <p><a class="btn btn-primary btn-large" href="{{ start_url }}">Get started &raquo;</a></p>
+
+		<form action="{{ start_url }}" method="post">
+		Name: <input type="text" name="participantid">
+		<input type="submit" value="Start">
+		</form>
+
         <p><div class="fb-like" data-layout="button_count" data-width="450" data-show-faces="false" data-font="arial"></div></p>
         <p><a href="https://twitter.com/share" class="twitter-share-button" data-via="RaymondJelierse" data-hashtags="vcd">Tweet</a></p>
     </header>