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/custom/src/Entity/Node/Talk.php

@@ -30,6 +30,14 @@ class Talk extends Node implements ContentEntityBundleInterface {
       ->referencedEntities());
   }
 
+  public function getNextDate(): ?int {
+    if ($this->get('field_event_date')->isEmpty()) {
+      return NULL;
+    }
+
+    return (int) $this->get('field_event_date')->getString();
+  }
+
   /**
    * Find the date for the latest event.
    *
@@ -42,4 +50,8 @@ class Talk extends Node implements ContentEntityBundleInterface {
       ->max();
   }
 
+  public function setNextDate(int $date): void {
+    $this->set('field_event_date', $date);
+  }
+
 }