internal_link: Open /near/ links at the given anchor in msglist

Fixes #82.

One TODO comment for this issue referred to an aspect I'm leaving out
of scope for now, namely when opening a notification rather than an
internal Zulip link.  So I've filed a separate issue #1565 for that,
and this updates that comment to point there.

Author: gnprice

lib/notifications/display.dart

@@ -513,7 +513,7 @@ class NotificationDisplayManager {
 
     return MessageListPage.buildRoute(
       accountId: account.id,
-      // TODO(#82): Open at specific message, not just conversation
+      // TODO(#1565): Open at specific message, not just conversation
       narrow: payload.narrow);
   }
 

lib/widgets/content.dart

@@ -1542,7 +1542,8 @@ void _launchUrl(BuildContext context, String urlString) async {
     case NarrowLink():
       unawaited(Navigator.push(context,
         MessageListPage.buildRoute(context: context,
-          narrow: internalLink.narrow)));
+          narrow: internalLink.narrow,
+          initAnchorMessageId: internalLink.nearMessageId)));
 
     case null:
       await PlatformActions.launchUrl(context, url);

test/widgets/content_test.dart

@@ -1032,6 +1032,8 @@ void main() {
         .page.isA<MessageListPage>().initNarrow.equals(const ChannelNarrow(1));
     });
 
+    // TODO(#1570): test links with /near/ go to the specific message
+
     testWidgets('invalid internal links are opened in browser', (tester) async {
       // Link is invalid due to `topic` operator missing an operand.
       final pushedRoutes = await prepare(tester,