Fix the ordering for future talks

Rather than the custom event sorting plugin being based on the `created`
value, this change adds a new `field_event_date` field to the talk node
type and uses this for the sorting instead.

This commit also adds a new `TalkDateUpdater` service that extracts
either the next event date if there is a future date, or the last past
event date if there is no future date, from `field_events` for each talk
and saves it into the event date field.

For consistency, and to ensure that the results are ordered correctly,
the talk date updater converts the date from a date string (e.g.
`2020-08-24`) into a UNIX timestamp, and the timestamp is saved in the
event date field. This can be changed at a later date if needed.

The talks view has been updated to use the updated sort plugin, and the
existing tests have been updated to use the new field.

References #204

web/modules/custom/opd_talks/src/Service/TalkDateUpdater.php

@@ -0,0 +1,71 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\opd_talks\Service;
+
+use Carbon\Carbon;
+use Drupal\Component\Datetime\TimeInterface;
+use Drupal\custom\Entity\Node\Talk;
+use Drupal\datetime\Plugin\Field\FieldType\DateTimeItemInterface;
+use Drupal\opd_talks\Repository\TalkRepository;
+use Drupal\paragraphs\ParagraphInterface;
+
+final class TalkDateUpdater {
+
+  private TalkRepository $talkRepository;
+  private TimeInterface $time;
+
+  public function __construct(
+    TalkRepository $talkRepository,
+    TimeInterface $time
+  ) {
+    $this->talkRepository = $talkRepository;
+    $this->time = $time;
+  }
+
+  public function __invoke(): void {
+    foreach ($this->talkRepository->getAll() as $talk) {
+      $this->updateNextEventDate($talk);
+    }
+  }
+
+  private function updateNextEventDate(Talk $talk) {
+    if (!$nextDate = $this->findNextEventDate($talk)) {
+      return;
+    }
+
+    $nextDateTimestamp = Carbon::parse($nextDate)
+      ->getTimestamp();
+
+    if ($nextDateTimestamp == $talk->getNextDate()) {
+      return;
+    }
+
+    $talk->setNextDate($nextDateTimestamp);
+    $talk->save();
+  }
+
+  private function findNextEventDate(Talk $talk): ?string {
+    $currentTime = Carbon::today()
+      ->format(DateTimeItemInterface::DATE_STORAGE_FORMAT);
+
+    $dates = $talk->getEvents()
+      ->map(fn(ParagraphInterface $event) => $event->get('field_date')
+        ->getString())
+      ->sort();
+
+    if ($dates->isEmpty()) {
+      return NULL;
+    }
+
+    // If a future date is found, return it.
+    if ($futureDate = $dates->first(fn(string $eventDate) => $eventDate > $currentTime)) {
+      return $futureDate;
+    }
+
+    // If no future date is found, return the last past date.
+    return $dates->last();
+  }
+
+}