Persistent Storage feature: time zone

In #15548 (closed), we made it possible for the user to set a correct time by setting their local time and time zone.

In #6284 (closed), we made it so the local time is displayed in the top navigation bar instead of the GMT time.

On top of this, we should allow storing this time zone in the Persistent Storage to avoid having people configure it each time.

From https://gitlab.tails.boum.org/tails/blueprints/-/wikis/network_connection#ux-design:

• Tor Connection assistant:

  

• Persistent Storage settings:

  

This persistent time zone can break stuff for users who switch between Linux (UTC) and Windows (local time) computers using the same Tails. If this happens they'll be encouraged to "fix your clock". We still want this persistent time zone to optimize for the majority of use cases (using computers that all have the same OS). And we should anyway make it easy to fix time zone issues, including this rare use case.

• Make sure that this persistent time zone is also applied when offline. See #19875 (closed).