Allow user specific timezones

[timroes]

The timezone setting in management is one of the settings, that could make more sense than others to have them user specific. I would suggest to be able to customize a timezone on a per user basis i.e. your browser session, not x-pack users necessarily. That way this timezone would only apply for you and not for all users.

The implementation could either mean having user specific settings, or - since I think there aren't too many that would make sense on a per user basis - just being able to overwrite the default timezone configured in management for you locally in the timepicker.

Derived from #18853, to be able to work around the date histogram time zone performance issue.

[timroes]

Closing this in favor of the more generic #17888