Timestamps shifted based on timezone for `wn_created_datetime` and `wn_modified_datetime`

[peterdesmet]

Our goal is to ignore all timezones for all timestamps, so timestamps never shift. This is now implemented throughout the whole data pipeline (sync -> db -> api -> export), except for:

• wn_created_datetime
• wn_modified_datetime

These values are not exposed through the Vespa-DB API, so it is not a big problem, but it would be good to fix those two as well, as they matter for the sync.

See #361 (comment) and #361 (comment) for examples.