[marlin-website] Add support for favicons

Summary:
This has been generated with realfavicongenerator.net so it supports all browsers and devices.

Depends on D19025.

Test Plan: Live at marlinwallet.com

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D19026