When available_time_zones or time_zones is not defined ... let user know what time zone.

[royendo]

Describe the bug
When available_time_zones or time_zones is not defined ... we default to UTC, but do not indicate so in the UI. When the paramater is defined, we add the local time zone as default.

To Reproduce
Deploy explore dashboard without time_zones defined or [deprecated dashboard with available_time_zones.

Expected behavior
It would help in the UI to have an indication of time zone. Whether selectable or not.

Screenshots
without parameter, no UTC indication.

With parameter but defaults to local time.