[e.cash] Add openInNewTab prop to /use-ecash buttons

Summary: Since these are external links they should open in new tabs

Test Plan: preview the site and check the links on /use-ecash open in new tabs

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D17334