Transactions merge should set `createdByUser` flag to false if one of the transactions is not created by user

The use case of manual transactions is often to plan a future transaction and then merge them together once the actual transactions happens.

When merging the transactions we should set its createdByUser flag to false if the actual transaction has been found.

See https://framagit.org/kresusapp/kresus/-/blob/main/server/models/helpers.ts#L23