Add support for format:2 transaction data

[DanMacDonald]

Arweave introduced a new format for transaction data but the weavemail app hasn't kept up.

Currently it is unable to view messages stored using the new transaction data format.

This PR adds support for format:2 transaction data and enables weavemail app to view messages again.

The behavior I'm observing is sometimes the weavemail transactions arrive with their txn.data field initialized and sometimes they don't. In the cases where .data is uninitialized the app isn't able to display the message. You'll see an error in the log about b64UrlToBuffer() unable to perform a .replace on the argument data because it's not a string.

This behavior is intermittent, some days the txn's have .data initialized sometimes they require .getData().