api: Add UserSettings.twentyFourHourTime

Not because we have an immediate need to read this user setting.
Rather, this demonstrates how much boilerplate it would take to
handle a new setting, with the setup from the previous commit.

Author: chrisbobbe

lib/api/model/events.dart

@@ -111,6 +111,8 @@ class UserSettingsUpdateEvent extends Event {
     switch (UserSettingName.fromRawString(json['property'] as String)) {
       case UserSettingName.displayEmojiReactionUsers:
         return value as bool;
+      case UserSettingName.twentyFourHourTime:
+        return value as bool;
       case null:
         return null;
     }

lib/api/model/events.g.dart

@@ -119,6 +119,7 @@ class RecentDmConversation {
 /// in <https://zulip.com/api/register-queue>.
 @JsonSerializable(fieldRename: FieldRename.snake, createFieldMap: true)
 class UserSettings {
+  bool twentyFourHourTime;
   bool? displayEmojiReactionUsers; // TODO(server-6)
 
   // TODO more, as needed. When adding a setting here, please also:
@@ -127,6 +128,7 @@ class UserSettings {
   // (3) handle the event that signals an update to the setting
 
   UserSettings({
+    required this.twentyFourHourTime,
     this.displayEmojiReactionUsers,
   });
 
@@ -148,6 +150,7 @@ class UserSettings {
 /// to ensure that every setting in [UserSettings] responds to the event.
 @JsonEnum(fieldRename: FieldRename.snake, alwaysCreate: true)
 enum UserSettingName {
+  twentyFourHourTime,
   displayEmojiReactionUsers;
 
   /// Get a [UserSettingName] from a raw, snake-case string we recognize, else null.

lib/api/model/initial_snapshot.dart

@@ -226,6 +226,8 @@ class PerAccountStore extends ChangeNotifier {
         return;
       }
       switch (event.property!) {
+        case UserSettingName.twentyFourHourTime:
+          userSettings?.twentyFourHourTime =        event.value as bool;
         case UserSettingName.displayEmojiReactionUsers:
           userSettings?.displayEmojiReactionUsers = event.value as bool;
       }