exp/lighthorizon: Properly transform transactions into JSON.

[Shaptic]

What

Properly transforms every field in a transaction record from XDR to JSON.

Why

Completeness and parity w/ Horizon Classic, closes #4527.

Known limitations

There's still a piece missing:

// TODO: Figure out what this means? Maybe @tamirms knows.
// if transactionHash != row.TransactionHash {
// 	dest.Signatures = dest.InnerTransaction.Signatures
// }

Any thoughts on this @tamirms? It comes from Horizon's code. I'm not sure what this check is, well, checking for...