[e.cash] Add video to about page

Summary: Replacing the image on the about page with a more engaging video and a cta to our youtube channel

Test Plan: preview and check /about page

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Subscribers: bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D19033