[Cashtab] Require ticker for token creation

Summary:
The spec supports creating without a ticker. However it is "better" to have a ticker, and Cashtab sorts tokens by their ticker on the tokens page.

Enforce ticker for users that mint with Cashtab.

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16807