ui: Follow user_settings.twenty_four_hour_time

[chrisbobbe]

When we show timestamps, for example on messages in the message list, we should format them according to the user's setting:

• user_settings.twenty_four_hour_time true: "13:52"
• user_settings.twenty_four_hour_time false: "1:52 PM"

See:

• Help center doc
• UserSettings.twentyFourHourTime in lib/api/model/initial_snapshot.dart
• UserSettingsUpdateEvent in lib/api/model/events.dart

Milestoning as "Launch" because I believe we do this in zulip-mobile.

[c-o-d-e-y-o-d-a]

I would like to work on this issue if it has not been assigned to anyone else.

[chrisbobbe]

Please see the Zulip project's guide to claiming an issue: https://zulip.readthedocs.io/en/latest/contributing/contributing.html#in-other-zulip-repositories

In particular:

In your comment, describe what part of the code you’re modifying and how you plan to approach the problem, based on what you learned in steps 1–4 above.