[FIX] website_event_track: use event timezone

Depending on the location, on the website different timezone were used
to display tracks:

- in `Agenda` day date (as of saas-14): the current user timezone
- in `Agenda` track times: the event timezone
- in `Talks` subtitle: the event timezone
- in `Talks` tracks: the current user timezone
- in a track page: the first user (admin) timezone

With this change in all those instance the event timezone is used.

There is still a usability hindrance in the backend: the timezone used
when encoding track or event datetime is the browser timezone.

So if the browser is GMT+2 and the event is GMT+6, encoding a 4h00 time
would save it as 2h00 (in UTC) and display it on the website as 8h00.

This would make sense to encode these datetime in user timezone but
currently there is no framework option to do it.

opw-805358
closes #22427

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