Update NSIS icons and imagery

Summary: Changes Bitcoin Core logos to Bitcoin-abc and Bitcoin Cash

Test Plan: make deploy, attach screenshots

Reviewers: #bitcoin_abc, O1 Bitcoin ABC, schancel, deadalnix

Reviewed By: #bitcoin_abc, O1 Bitcoin ABC, schancel, deadalnix

Subscribers: schancel, deadalnix, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1373