refs https://github.com/TryGhost/Ghost/pull/7142, https://github.com/TryGhost/Ghost/pull/7143
- moves timezone selection template and logic into a component
- detect if the current `activeTimezone` is not in our pre-defined list of timezones, if it isn't:
- add a line indicating that there has been an override with the current `activeTimezone` value
- add a blank option to the timezone select list