Add an "author" field to proposal_object to remove ambiguity in case both creator and receiver got a post with the same permlink

This is an issue because we currently only have a permlink field to fetch the linked post and we allow both creator and receiver to have the linked permlink:

      const auto* commentObject = _db.find_comment(o.creator, o.permlink);
      if(commentObject == nullptr)
      {
         commentObject = _db.find_comment(o.receiver, o.permlink);
         FC_ASSERT(commentObject != nullptr, "Proposal permlink must point to the article posted by creator or receiver");
      }

But if alice and bob both have a post with the same permlink will leads to ambiguity and the front end won't know which post to display, potentially the wrong one.