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/field.field.node.talk.field_event_date.yml

@@ -0,0 +1,23 @@
+uuid: 5c54e34a-4e53-4e70-b621-1d40953385cd
+langcode: en
+status: true
+dependencies:
+  config:
+    - field.storage.node.field_event_date
+    - node.type.talk
+id: node.talk.field_event_date
+field_name: field_event_date
+entity_type: node
+bundle: talk
+label: 'Next event date'
+description: ''
+required: false
+translatable: false
+default_value: {  }
+default_value_callback: ''
+settings:
+  min: 0
+  max: null
+  prefix: ''
+  suffix: ''
+field_type: integer