[token-server] Support creating tokens with icons from locally run cashtab

Summary: The pain-in-the-ass of forgetting this is not supported while testing is imo more impactful than the potential ddos vector of someone hitting this API from localhost:3000

Test Plan: npm test, will need to test with local cashtab after this is live but it does work locally

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Subscribers: emack

Differential Revision: https://reviews.bitcoinabc.org/D16033