T2972
See D14030. This change is needed to support more complicated parsing of OP_RETURN msgs, i.e. if some of the data could be used to generate a link in the tg msg.
Currently, ecash-herald calls prepareStringForTelegramHTML on the whole msg for OP_RETURN txs. However, we want this to depend on the msg contents.
At this point the app doesn't support any complicated OP_RETURN pushes so it's more or less a non-change. D14030 shows how this can be used to create better msgs for OP_RETURN txs.