[e.cash] Fix twitter image tag

Summary: Adding the twitter:image meta tag to the head to resolve the image not showing up when shared

Test Plan: After the change is live try sharing the e.cash url on twitter and see if the image shows. (their card validator no longer works afaik)

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D14428