channel: Add selfHasContentAccess, using canSubscribeGroup et al

Fixes zulip#814.

The general implementation of group-based permissions was added in
the commits leading up to this one.  This commit demonstrates
implementing a given such permission.

This particular permission is one we don't yet have any logic that
should consume; we'll be adding that as part of zulip#188, where we'll want
to know whether to offer the option to subscribe to a given channel.

To exercise this method on real data, I added some debugging prints
that printed a list of the channels where this returned true, and a
list of the channels where it returned false.

Author: gnprice

lib/model/channel.dart

@@ -5,6 +5,7 @@ import 'package:flutter/foundation.dart';
 import '../api/model/events.dart';
 import '../api/model/initial_snapshot.dart';
 import '../api/model/model.dart';
+import 'realm.dart';
 import 'store.dart';
 import 'user.dart';
 
@@ -139,6 +140,40 @@ mixin ChannelStore on UserStore {
     }
   }
 
+  bool selfHasContentAccess(ZulipStream channel) {
+    // Compare web's stream_data.has_content_access.
+    if (channel.isWebPublic) return true;
+    if (channel is Subscription) return true;
+    // Here web calls has_metadata_access... but that always returns true,
+    // as its comment says.
+    if (selfUser.role == UserRole.guest) return false;
+    if (!channel.inviteOnly) return true;
+    return _selfHasContentAccessViaGroupPermissions(channel);
+  }
+
+  bool _selfHasContentAccessViaGroupPermissions(ZulipStream channel) {
+    // Compare web's stream_data.has_content_access_via_group_permissions.
+    if (channel.canAddSubscribersGroup != null
+        && selfHasPermissionForGroupSetting(channel.canAddSubscribersGroup!,
+             GroupSettingType.stream, 'can_add_subscribers_group')) {
+      // The behavior before this permission was introduced was equivalent to
+      // the "nobody" group.
+      // TODO(server-10): simplify
+      return true;
+    }
+
+    if (channel.canSubscribeGroup != null
+        && selfHasPermissionForGroupSetting(channel.canSubscribeGroup!,
+             GroupSettingType.stream, 'can_subscribe_group')) {
+      // The behavior before this permission was introduced was equivalent to
+      // the "nobody" group.
+      // TODO(server-10): simplify
+      return true;
+    }
+
+    return false;
+  }
+
   bool hasPostingPermission({
     required ZulipStream inChannel,
     required User user,