Fix the discussionThread id type

metadata-protobuf/proto/Bounty.proto

@@ -3,7 +3,7 @@ syntax = "proto2";
 message BountyMetadata {
   optional string title = 1; // Bounty title
   optional string description = 2; // Bounty description
-  optional uint32 discussionThread = 3; // Id of the forum thread used to discuss the bounty
+  optional uint64 discussionThread = 3; // Id of the forum thread used to discuss the bounty
   optional string banner_image_uri = 4; // Image uri of the bounty's banner
 }