[Cashtab] Render paywall id directly as hex

Summary:
Following on from D16623:

I have corrected this on eCashChat's side so the opreturn script has the article txid in hex form...

This diff updates Cashtab to render the paywall article txid directly from hex format

Test Plan: npm test

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D16558