internal_link: Generate narrow links with "channel", not "stream"

Fixes: zulip#633

Author: sm-sayedi

lib/api/model/narrow.dart

@@ -35,7 +35,7 @@ sealed class ApiNarrowElement {
 }
 
 class ApiNarrowStream extends ApiNarrowElement {
-  @override String get operator => 'stream';
+  @override String get operator => 'channel';
 
   @override final int operand;
 

test/model/compose_test.dart

@@ -248,22 +248,22 @@ hello
           .equals(store.realmUrl.resolve(expectedFragment));
       }
 
-      checkNarrow(streamId: 1,   name: 'announce',       '#narrow/stream/1-announce');
-      checkNarrow(streamId: 378, name: 'api design',     '#narrow/stream/378-api-design');
-      checkNarrow(streamId: 391, name: 'Outreachy',      '#narrow/stream/391-Outreachy');
-      checkNarrow(streamId: 415, name: 'chat.zulip.org', '#narrow/stream/415-chat.2Ezulip.2Eorg');
-      checkNarrow(streamId: 419, name: 'français',       '#narrow/stream/419-fran.C3.A7ais');
-      checkNarrow(streamId: 403, name: 'Hshs[™~}(.',     '#narrow/stream/403-Hshs.5B.E2.84.A2~.7D.28.2E');
-      checkNarrow(streamId: 60,  name: 'twitter', nearMessageId: 1570686, '#narrow/stream/60-twitter/near/1570686');
+      checkNarrow(streamId: 1,   name: 'announce',       '#narrow/channel/1-announce');
+      checkNarrow(streamId: 378, name: 'api design',     '#narrow/channel/378-api-design');
+      checkNarrow(streamId: 391, name: 'Outreachy',      '#narrow/channel/391-Outreachy');
+      checkNarrow(streamId: 415, name: 'chat.zulip.org', '#narrow/channel/415-chat.2Ezulip.2Eorg');
+      checkNarrow(streamId: 419, name: 'français',       '#narrow/channel/419-fran.C3.A7ais');
+      checkNarrow(streamId: 403, name: 'Hshs[™~}(.',     '#narrow/channel/403-Hshs.5B.E2.84.A2~.7D.28.2E');
+      checkNarrow(streamId: 60,  name: 'twitter', nearMessageId: 1570686, '#narrow/channel/60-twitter/near/1570686');
 
       checkNarrow(streamId: 48,  name: 'mobile', topic: 'Welcome screen UI',
-                  '#narrow/stream/48-mobile/topic/Welcome.20screen.20UI');
+                  '#narrow/channel/48-mobile/topic/Welcome.20screen.20UI');
       checkNarrow(streamId: 243, name: 'mobile-team', topic: 'Podfile.lock clash #F92',
-                  '#narrow/stream/243-mobile-team/topic/Podfile.2Elock.20clash.20.23F92');
+                  '#narrow/channel/243-mobile-team/topic/Podfile.2Elock.20clash.20.23F92');
       checkNarrow(streamId: 377, name: 'translation/zh_tw', topic: '翻譯 "stream"',
-                  '#narrow/stream/377-translation.2Fzh_tw/topic/.E7.BF.BB.E8.AD.AF.20.22stream.22');
+                  '#narrow/channel/377-translation.2Fzh_tw/topic/.E7.BF.BB.E8.AD.AF.20.22stream.22');
       checkNarrow(streamId: 42,  name: 'Outreachy 2016-2017', topic: '2017-18 Stream?', nearMessageId: 302690,
-                  '#narrow/stream/42-Outreachy-2016-2017/topic/2017-18.20Stream.3F/near/302690');
+                  '#narrow/channel/42-Outreachy-2016-2017/topic/2017-18.20Stream.3F/near/302690');
     });
 
     test('DmNarrow', () {
@@ -344,11 +344,11 @@ hello
     await store.addUser(sender);
 
     check(quoteAndReplyPlaceholder(store, message: message)).equals('''
-@_**Full Name|123** [said](${eg.selfAccount.realmUrl}#narrow/stream/1-test-here/topic/some.20topic/near/${message.id}): *(loading message ${message.id})*
+@_**Full Name|123** [said](${eg.selfAccount.realmUrl}#narrow/channel/1-test-here/topic/some.20topic/near/${message.id}): *(loading message ${message.id})*
 ''');
 
     check(quoteAndReply(store, message: message, rawContent: 'Hello world!')).equals('''
-@_**Full Name|123** [said](${eg.selfAccount.realmUrl}#narrow/stream/1-test-here/topic/some.20topic/near/${message.id}):
+@_**Full Name|123** [said](${eg.selfAccount.realmUrl}#narrow/channel/1-test-here/topic/some.20topic/near/${message.id}):
 ```quote
 Hello world!
 ```