[e.cash] Open external button links in new tabs

Summary: A few more that were missed in D17334

Test Plan:
git grep button to find all the places the button is used
Run the site locally to check that the external links open in a new tab

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D17346