wip/link and fetch at first unread from widget, behind flag; TODO a real setting

TODO: for me, in combined feed, this is a lot slower than fetching
  newest -- seems like the server request is a lot slower,
  which makes sense.
  So I want to just always start at newest in, I guess, interleaved views.

Fixes 80.

Author: gnprice

lib/model/settings.dart

@@ -117,6 +117,10 @@ enum BoolGlobalSetting {
   /// errors are encountered.
   forceRenderKatex(GlobalSettingType.experimentalFeatureFlag, false),
 
+  /// An experimental flag to open the message list at the first unread,
+  /// instead of the newest messages.
+  openFirstUnread(GlobalSettingType.experimentalFeatureFlag, false),
+
   // Former settings which might exist in the database,
   // whose names should therefore not be reused:
   // (this list is empty so far)

lib/widgets/message_list.dart

@@ -7,6 +7,7 @@ import '../api/model/model.dart';
 import '../generated/l10n/zulip_localizations.dart';
 import '../model/message_list.dart';
 import '../model/narrow.dart';
+import '../model/settings.dart';
 import '../model/store.dart';
 import '../model/typing_status.dart';
 import 'action_sheet.dart';
@@ -246,9 +247,14 @@ class _MessageListPageState extends State<MessageListPage> implements MessageLis
         actions.add(_TopicListButton(streamId: streamId));
     }
 
-    // TODO(#80): default to anchor firstUnread, instead of newest
-    final initAnchor = widget.initAnchorMessageId == null
-      ? AnchorCode.newest : NumericAnchor(widget.initAnchorMessageId!);
+    final Anchor initAnchor;
+    if (widget.initAnchorMessageId != null) {
+      initAnchor = NumericAnchor(widget.initAnchorMessageId!);
+    } else {
+      final globalSettings = GlobalStoreWidget.settingsOf(context);
+      initAnchor = globalSettings.getBool(BoolGlobalSetting.openFirstUnread)
+        ? AnchorCode.firstUnread : AnchorCode.newest;
+    }
 
     // Insert a PageRoot here, to provide a context that can be used for
     // MessageListPage.ancestorOf.