[website] Add Kucoin back to list of exchanges

Summary: BCHA withdrawal has been re-enabled, so add them back to the list

Test Plan:
Confirmed with several sources who have accounts there that withdrawal is in fact enabled
Built and ran site locally, checked image rendering and link.

Reviewers: josephroyking, #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9274