Show upcoming talks in chronological order

Update the ordering of the future talks on the talks page so that
upcoming talks are shown in chronological order (soonest first),
followed by past talks in reverse chronological order (most recent
first).

This still uses the `created` date for ordering, which is updated
automatically on saving the node to match the furthest future talk, but
I may want to move that into a custom node property at some point.

Fixes #140

web/modules/custom/opd_talks/src/Plugin/views/sort/Event.php

@@ -0,0 +1,40 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\opd_talks\Plugin\views\sort;
+
+use Drupal\Core\Datetime\DrupalDateTime;
+use Drupal\datetime\DateTimeComputed;
+use Drupal\views\Plugin\views\sort\Date;
+use Drupal\views\Annotation\ViewsSort;
+
+/**
+ * @ViewsSort("event_sort")
+ */
+final class Event extends Date {
+
+   public function query() {
+     $this->ensureMyTable();
+
+     $currentTime = time();
+     $dateAlias = "$this->tableAlias.$this->realField";
+
+     // Is this event in the past?
+     $this->query->addOrderBy(
+       NULL,
+       sprintf("%d > %s", $currentTime, $dateAlias),
+       $this->options['order'],
+       "in_past"
+     );
+
+     // How far in the past/future is this event?
+     $this->query->addOrderBy(
+       NULL,
+       sprintf('ABS(%s - %d)', $dateAlias, $currentTime),
+       $this->options['order'],
+       "distance_from_now"
+     );
+   }
+
+}