[e.cash] Add /get-ecash link to navbar price box

Summary: This was removed until the page was ready. Adding it back now

Test Plan: preview the site and check the link works

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Subscribers: bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D14328