Resolve the Local time zone in the UI

[begelundmuller]

In the explore and canvas YAML, we allow a time zone called Local. However if you pass it directly to the metrics APIs, it always resolves to UTC because it's resolved using the server's time zone, not the browser's. Example YAML:

time_zones:
- UTC
- Local
- Europe/Copenhagen

To support the browser's local time, the UI should resolve Local to the IANA code of browser's local time and send the resolved IANA code in metrics API calls instead.