model: Add hasPostingPermission helper method to ZulipStream

Author: sm-sayedi

lib/api/model/model.dart

@@ -372,6 +372,19 @@ class ZulipStream {
     _$ZulipStreamFromJson(json);
 
   Map<String, dynamic> toJson() => _$ZulipStreamToJson(this);
+
+  bool hasPostingPermission(User user, {required int realmWaitingPeriodThreshold}) {
+    final role = user.role;
+    return switch (channelPostPolicy) {
+      ChannelPostPolicy.any            => true,
+      ChannelPostPolicy.fullMembers    => role != UserRole.guest && (role == UserRole.member
+                                            ? user.isFullMember(realmWaitingPeriodThreshold)
+                                            : true),
+      ChannelPostPolicy.moderators     => role != UserRole.guest && role != UserRole.member,
+      ChannelPostPolicy.administrators => role == UserRole.administrator || role == UserRole.owner || role == UserRole.unknown,
+      ChannelPostPolicy.unknown        => true,
+    };
+  }
 }
 
 /// Policy for which users can post to the stream.