api [nfc]: Reorder and rearrange Subscription fields to parallel Stream

This type is long enough to kind of push the limit for manually
comparing the docs to the type when they're not in the same order.
But in practice for future updates to it I think we can rely on
reading through its API docs for "Changes" notes.

And meanwhile, even within the API docs the `subscriptions` and the
`streams` lists don't agree on the order of the fields, even among
those they have in common.  So we might as well give up on matching
them and go for a logical order instead.

Author: gnprice

lib/api/model/model.dart

@@ -173,15 +173,33 @@ class ZulipStream {
 /// in <https://zulip.com/api/register-queue>.
 @JsonSerializable(fieldRename: FieldRename.snake)
 class Subscription {
+  // First, fields that are about the stream and not the user's relation to it.
+  // These are largely the same as in [ZulipStream].
+
   final int streamId;
   final String name;
   final String description;
   final String renderedDescription;
+
   final int dateCreated;
-  final bool inviteOnly;
+  final int? firstMessageId;
+  final int? streamWeeklyTraffic;
 
+  final bool inviteOnly;
+  final bool? isWebPublic; // TODO(server-??): doc doesn't say when added
+  final bool historyPublicToSubscribers;
+  final int? messageRetentionDays;
   // final List<int> subscribers; // we register with includeSubscribers false
 
+  final int streamPostPolicy; // TODO enum
+  // final bool? isAnnouncementOnly; // deprecated; ignore
+  final String emailAddress;
+
+  final int? canRemoveSubscribersGroupId; // TODO(server-6)
+
+  // Then, fields that are specific to the subscription,
+  // i.e. the user's relationship to the stream.
+
   final bool? desktopNotifications;
   final bool? emailNotifications;
   final bool? wildcardMentionsNotify;
@@ -190,26 +208,11 @@ class Subscription {
 
   final bool pinToTop;
 
-  final String emailAddress;
-
   final bool isMuted;
-
   // final bool? inHomeView; // deprecated; ignore
 
-  // final bool? isAnnouncementOnly; // deprecated; ignore
-  final bool? isWebPublic; // TODO(server-??): doc doesn't say when added
-
   final String color;
 
-  final int streamPostPolicy; // TODO enum
-  final int? messageRetentionDays;
-  final bool historyPublicToSubscribers;
-
-  final int? firstMessageId;
-  final int? streamWeeklyTraffic;
-
-  final int? canRemoveSubscribersGroupId; // TODO(server-6)
-
   Subscription({
     required this.streamId,
     required this.name,