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/opd_talks.services.yml

@@ -1,3 +1,6 @@
 services:
   Drupal\opd_talks\Repository\TalkRepository:
     autowire: true
+
+  Drupal\opd_talks\Service\TalkDateUpdater:
+    autowire: true