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/tests/src/Kernel/TalkEventDateTest.php

@@ -0,0 +1,104 @@
+<?php
+
+namespace Drupal\Tests\opd_talks\Kernel;
+
+use Carbon\Carbon;
+use Drupal\custom\Entity\Node\Talk;
+use Drupal\datetime\Plugin\Field\FieldType\DateTimeItemInterface;
+use Drupal\node\Entity\Node;
+use Drupal\opd_talks\Service\TalkDateUpdater;
+use Drupal\Tests\custom\Kernel\TalksTestBase;
+
+final class TalkEventDateTest extends TalksTestBase {
+
+  /** @test */
+  public function talk_event_dates_are_set_to_the_next_future_date(): void {
+    $dateFormat = DateTimeItemInterface::DATE_STORAGE_FORMAT;
+
+    $talk = $this->createTalk([
+      'field_event_date' => NULL,
+      'field_events' => [
+        $this->createEvent([
+          'field_date' => Carbon::today()
+            ->subWeeks(2)
+            ->format($dateFormat),
+        ]),
+        $this->createEvent([
+          'field_date' => Carbon::today()
+            ->subDays(2)
+            ->format($dateFormat),
+        ]),
+        $this->createEvent([
+          'field_date' => Carbon::today()
+            ->addDays(4)
+            ->format($dateFormat),
+        ]),
+        $this->createEvent([
+          'field_date' => Carbon::today()
+            ->addDays(10)
+            ->format($dateFormat),
+        ]),
+      ],
+    ]);
+
+    $dateUpdater = $this->container->get(TalkDateUpdater::class);
+    $dateUpdater->__invoke();
+
+    $expected = Carbon::today()->addDays(4)->getTimestamp();
+
+    $talk = Node::load($talk->id());
+    $this->assertNextEventDateIs($talk, $expected);
+  }
+
+  /** @test */
+  public function talk_event_dates_are_set_to_the_last_past_date(): void {
+    $dateFormat = DateTimeItemInterface::DATE_STORAGE_FORMAT;
+
+    $talk = $this->createTalk([
+      'field_event_date' => NULL,
+      'field_events' => [
+        $this->createEvent([
+          'field_date' => Carbon::today()
+            ->subDays(4)
+            ->format($dateFormat),
+        ]),
+        $this->createEvent([
+          'field_date' => Carbon::today()
+            ->subDays(2)
+            ->format($dateFormat),
+        ]),
+      ],
+    ]);
+
+    $dateUpdater = $this->container->get(TalkDateUpdater::class);
+    $dateUpdater->__invoke();
+
+    $expected = Carbon::today()->subDays(2)->getTimestamp();
+
+    $talk = Node::load($talk->id());
+    $this->assertNextEventDateIs($talk, $expected);
+  }
+
+  /** @test */
+  public function next_event_date_is_empty_if_there_are_no_events(): void {
+    $talk = $this->createTalk([
+      'field_event_date' => NULL,
+      'field_events' => [],
+    ]);
+
+    $dateUpdater = $this->container->get(TalkDateUpdater::class);
+    $dateUpdater->__invoke();
+
+    $talk = Node::load($talk->id());
+    $this->assertNoNextEventDate($talk);
+  }
+
+  private function assertNextEventDateIs(Talk $talk, $expected): void {
+    $this->assertSame($expected, $talk->getNextDate());
+  }
+
+  private function assertNoNextEventDate(Talk $talk): void {
+    $this->assertNull($talk->getNextDate());
+  }
+
+}