[e.cash] Add extension to wallet page

Summary: Add the cashtab extension to the wallet page otherwise it's prone to users googling up a scam wallet.

Test Plan: preview the wallet page, click on the extension link

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D15656