Implement drupal_get_user_timezone()

[KarinG]

Before

Contributions made by anonymous user were recorded in CiviCRM using timezone set on server level.

Drupal8.php is the only file in /var/www/drupal/vendor/civicrm/civicrm-core/CRM/Utils/System without a public function getTimeZoneString()

After

After implementing drupal_get_user_timezone() -> Contributions made by anonymous users are now recorded in the Default timezone configured within Drupal8:
/admin/config/regional/settings

For testing I added a drupal_set_message(print_r($timezone, TRUE)) in Drupal8.php -> it shows it's correctly finding America/Edmonton.

Technical Details

Similar to the D7 implementation - except simpler. Too simple actually - I could well be missing something though I did some more testing and this seems to work.

Todo: more Testing -> done:

1. on /admin/config/regional/settings-> change default TimeZone in Drupal to Config to -> Europe/Amsterdam
2. Anonymous user is now recognized in that timezone (see drupal_set_message left bottom)
   
3. And the Contribution in CiviCRM is correctly recorded in that site's default TimeZone - which is indeed +8h ->
   
4. Note for all testing above -> php -i |grep timezone -> UTC

[civibot]

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[mlutfy]

Merging based on code review, mattermost discussions and testing on a live site (although admittedly not extensively, but I trust that Karin has).