channel [nfc]: Add HasChannelStore for other substores to use

Like HasUserStore for data about users (added in 623bcb4), and
HasRealmStore for data about the realm, this will help make it
convenient for other substores to refer to data about channels.

Author: chrisbobbe

lib/model/channel.dart

@@ -16,6 +16,9 @@ import 'user.dart';
 ///
 /// The data structures described here are implemented at [ChannelStoreImpl].
 mixin ChannelStore on UserStore {
+  @protected
+  UserStore get userStore;
+
   /// All known channels/streams, indexed by [ZulipStream.streamId].
   ///
   /// The same [ZulipStream] objects also appear in [streamsByName].
@@ -248,6 +251,18 @@ mixin ProxyChannelStore on ChannelStore {
     channelStore.debugTopicVisibility;
 }
 
+/// A base class for [PerAccountStore] substores
+/// that need access to [ChannelStore] as well as to its prerequisites
+/// [CorePerAccountStore], [RealmStore], and [UserStore].
+abstract class HasChannelStore extends HasUserStore with ChannelStore, ProxyChannelStore {
+  HasChannelStore({required ChannelStore channels})
+    : channelStore = channels, super(users: channels.userStore);
+
+  @protected
+  @override
+  final ChannelStore channelStore;
+}
+
 /// The implementation of [ChannelStore] that does the work.
 ///
 /// Generally the only code that should need this class is [PerAccountStore]