Adding and deleting time zones

[oz]

The UI must allow one to add arbitrary time zones from the tz db.

• key bindings to navigate between the configured time zones (arrows, j, k, ...),
• delete, or backspace, or d should show a confirmation dialog to remove the zone,
• + to show the "add a time zone" dialog where the UI helps auto-complete from the tzdb list.

Notes:

Since go 1.15, there is a time/tzdata package that one can embed to work around cases where the info is not available on the host (this adds 800kb to the binary 🐘).

[oz]

Duh. Let's just use env. variables for now.