[explorer] Add backlink to e.cash in footer
Summary: Adding a link to e.cash in the footer to help with SEO/provide navigation to that site
Test Plan: Run the app and see if its there and looks good. And let me know if any other ideas for a good place to put it
Reviewers: #bitcoin_abc, Mengerian
Reviewed By: #bitcoin_abc, Mengerian
Subscribers: Kronkmeister, Mengerian
Differential Revision: https://reviews.bitcoinabc.org/D13245