Scheduler time zone addons + a11y improvements for form elements.

[amazingphilippe]

Summary | Résumé

This PR adds precisions about time-zone when scheduling messages. It also improves general a11y of the scheduler by

• Adding a functioning label and hint to the time selector
• Adding a functioning label and proper markup to the ampm/24h radios
• Fix a bug with the announcer (was inverted when the page was in FR)

Test instructions | Instructions pour tester la modification

1. When sending in bulk, on the review screen
2. Open the scheduler
3. Inspect the Announcer area near the calendar
4. Toggle the time format, test in FR and EN
5. Inspect the confirmation message in yellow
6. Toggle the time format, text in FR and EN
7. Use VoiceOver, or inspect aria properties of the time selector and the time format radios.

Todo:

• [ ] Validate translations with @mariesohiebezert
• [ ] Maybe hide the label for the time format radios: redundant for sighted users.