channel [nfc]: Rename file from "stream" to "channel"

Fixes parts of #631

Author: Khader-1

lib/model/stream.dart renamed to lib/model/channel.dart

@@ -4,7 +4,7 @@ import 'package:json_annotation/json_annotation.dart';
 import '../api/model/narrow.dart';
 import 'narrow.dart';
 import 'store.dart';
-import 'stream.dart';
+import 'channel.dart';
 
 part 'internal_link.g.dart';
 

lib/model/internal_link.dart

@@ -23,7 +23,7 @@ import 'database.dart';
 import 'message.dart';
 import 'message_list.dart';
 import 'recent_dm_conversations.dart';
-import 'stream.dart';
+import 'channel.dart';
 import 'typing_status.dart';
 import 'unreads.dart';
 

lib/model/store.dart

@@ -9,7 +9,7 @@ import '../api/model/events.dart';
 import '../log.dart';
 import 'algorithms.dart';
 import 'narrow.dart';
-import 'stream.dart';
+import 'channel.dart';
 
 /// The view-model for unread messages.
 ///

lib/model/unreads.dart

@@ -5,7 +5,7 @@ import 'package:zulip/api/model/events.dart';
 import 'package:zulip/api/model/initial_snapshot.dart';
 import 'package:zulip/api/model/model.dart';
 import 'package:zulip/model/store.dart';
-import 'package:zulip/model/stream.dart';
+import 'package:zulip/model/channel.dart';
 
 import '../example_data.dart' as eg;
 import 'test_store.dart';