refactor: Remove the sortable_date for talks

Automatically calculate the sortable date for a talk using the event
dates. This means that the `sortable_date` in the YAML front matter can
be removed.

Fixes #4

src/TwigExtension/TalkExtension.php

@@ -5,6 +5,7 @@ declare(strict_types=1);
 namespace App\TwigExtension;
 
 use App\Collection\TalkCollection;
+use Illuminate\Support\Collection;
 use Twig\Extension\AbstractExtension;
 use Twig\TwigFunction;
 
@@ -13,10 +14,18 @@ final class TalkExtension extends AbstractExtension
     public function getFunctions()
     {
         return [
+            new TwigFunction('get_last_event_date_for_talk', [$this, 'getLastEventDate']),
             new TwigFunction('get_past_talk_count', [$this, 'getPastTalkCount']),
         ];
     }
 
+    public function getLastEventDate($talk): ?string
+    {
+        $events = new Collection($talk['events']);
+
+        return $events->pluck('date')->sort()->last();
+    }
+
     public function getPastTalkCount(iterable $talks = []): int
     {
         return $this->getEventsFromTalks($talks)->count();