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

config/default/views.view.talks.yml

@@ -144,7 +144,7 @@ display:
       sorts:
         event_sort:
           id: event_sort
-          table: node_field_data
+          table: node__field_event_date
           field: event_sort
           relationship: none
           group_type: group
@@ -154,7 +154,6 @@ display:
           expose:
             label: ''
           granularity: second
-          entity_type: node
           plugin_id: event_sort
       title: Talks
       header: {  }