[Backport to release/4.0] - Allow to set the users timezone

There doesn't seem to be a way to set the timezone within the CMS or for the user?

When a user is in a different timezone than the system default the time in date pickers is always "wrong".

Ideally timezone would be user specific setting, as it's feasible, especially in the USA, that users of the CMS are not just in a different timezone than the system, but different timezones from each other.