api: Add InitialSnapshot.userSettings, with one user setting to start

This will make [displayEmojiReactionUsers] available for zulip#121,
showing message reactions. (Though it won't be live-updated yet;
we'll do that soon.)

Related: zulip#121

Author: chrisbobbe

lib/api/model/initial_snapshot.dart

@@ -30,6 +30,14 @@ class InitialSnapshot {
 
   final List<ZulipStream> streams;
 
+  // Servers pre-5.0 don't have `user_settings`, and instead provide whatever
+  // user settings they support at toplevel in the initial snapshot. Since we're
+  // likely to desupport pre-5.0 servers before wide release, we prefer to
+  // ignore the toplevel fields and use `user_settings` where present instead,
+  // even at the expense of functionality with pre-5.0 servers.
+  // TODO(server-5) remove pre-5.0 comment
+  final UserSettings? userSettings; // TODO(server-5)
+
   final int maxFileUploadSizeMib;
 
   @JsonKey(readValue: _readUsersIsActiveFallbackTrue)
@@ -71,6 +79,7 @@ class InitialSnapshot {
     required this.recentPrivateConversations,
     required this.subscriptions,
     required this.streams,
+    required this.userSettings,
     required this.maxFileUploadSizeMib,
     required this.realmUsers,
     required this.realmNonActiveUsers,
@@ -102,3 +111,23 @@ class RecentDmConversation {
 
   Map<String, dynamic> toJson() => _$RecentDmConversationToJson(this);
 }
+
+/// The `user_settings` dictionary.
+///
+/// For docs, search for "user_settings:"
+/// in <https://zulip.com/api/register-queue>.
+@JsonSerializable(fieldRename: FieldRename.snake)
+class UserSettings {
+  final bool? displayEmojiReactionUsers; // TODO(server-6)
+
+  // TODO more, as needed
+
+  UserSettings({
+    required this.displayEmojiReactionUsers,
+  });
+
+  factory UserSettings.fromJson(Map<String, dynamic> json) =>
+    _$UserSettingsFromJson(json);
+
+  Map<String, dynamic> toJson() => _$UserSettingsToJson(this);
+}

lib/api/model/initial_snapshot.g.dart

@@ -208,6 +208,7 @@ InitialSnapshot initialSnapshot({
   List<RecentDmConversation>? recentPrivateConversations,
   List<Subscription>? subscriptions,
   List<ZulipStream>? streams,
+  UserSettings? userSettings,
   int? maxFileUploadSizeMib,
   List<User>? realmUsers,
   List<User>? realmNonActiveUsers,
@@ -224,6 +225,7 @@ InitialSnapshot initialSnapshot({
     recentPrivateConversations: recentPrivateConversations ?? [],
     subscriptions: subscriptions ?? [], // TODO add subscriptions to default
     streams: streams ?? [], // TODO add streams to default
+    userSettings: userSettings, // TODO add userSettings to default
     maxFileUploadSizeMib: maxFileUploadSizeMib ?? 25,
     realmUsers: realmUsers ?? [],
     realmNonActiveUsers: realmNonActiveUsers ?? [],