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.storage.node.field_event_date.yml

@@ -0,0 +1,20 @@
+uuid: 86aec221-e3cb-4da3-90b5-522b661c6313
+langcode: en
+status: true
+dependencies:
+  module:
+    - node
+id: node.field_event_date
+field_name: field_event_date
+entity_type: node
+type: integer
+settings:
+  unsigned: false
+  size: normal
+module: core
+locked: false
+cardinality: 1
+translatable: true
+indexes: {  }
+persist_with_no_fields: false
+custom_storage: false