Fix timezone interface

The current timezone-selector interface is a bit clunky and broken. The GMT-offsets (technically UTC-offsets since GMT 00:00 is noon and midnight) are listed as the offset-as-of-january-1; since many timezones are currently observing DST, these offsets don't reflect the actual offsets currently, and since DST transitions are not consistent from timezone-to-timezone, they can't be reliably used by users in different regions to agree on a time for pairing.

A much more straight-forward and lossless interface would be to allow users to pick from the list of "#{contintent}/#{example_city}" list (plus the Etc/UTC entry), which are timezones that include DST transitions where necessary; these timezones can then be used to output any timestamp in its currently-active UTC-offset and are unambiguous without assuming context (e.g., Central Time – Central US? Central Australia? Central Europe?).