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/core.entity_view_display.node.talk.teaser.yml

@@ -5,6 +5,7 @@ dependencies:
   config:
     - core.entity_view_mode.node.teaser
     - field.field.node.talk.body
+    - field.field.node.talk.field_event_date
     - field.field.node.talk.field_events
     - field.field.node.talk.field_excerpt
     - field.field.node.talk.field_slides
@@ -32,6 +33,7 @@ content:
     third_party_settings: {  }
 hidden:
   body: true
+  field_event_date: true
   field_events: true
   field_slides: true
   field_type: true