[Cashtab] Prevent token creation with ticker MUSD or name Marianas U.S. Dollar

Summary: The real one already exists. Scams are potentially inevitable but ideally should not come from Cashtab.

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18102