[Cashtab] move newTokenDocumentUrl to Ticker.js

Summary: removed hardcoded cashtab.com fromt CreateTokenForm.js, replaced with parameter called from Ticker.js currency obj

Test Plan:
cd web/cashtab
npm start
mint token, leaving url blank,
ensure auto generated url is correct

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D10532