R1:efd001b42fd5

Wordsmith the timezone selection UX

Summary:
Ref T3025.

- Show current zone to make the current vs new more clear.
- Tweak some text.

Test Plan: {F1656534}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T3025

Differential Revision: https://secure.phabricator.com/D15965