Add participant ID

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

views/results.html.twig

@@ -54,6 +54,7 @@
                 <th>uid</th>
                 <th>date</th>
                 <th>host</th>
+                <th>participant</th>
                 <th>phase</th>
                 <th>x</th>
                 <th>y</th>
@@ -67,6 +68,7 @@
                 <td>{{record.uid}}.</td>
                 <td>{{ record.datetime }}</td>
                 <td>{{ record.host }}</td>
+                <td>{{ record.participantid }}</td>                
                 <td>{{ record.phase }}</td>
                 <td>{{ record.xcoordinate }}</td>
                 <td>{{ record.ycoordinate }}</td>