[Cashtab] Add XBT to banned token ticker array

Summary: Related to T2829. Added xbt to banned token ticker array.

Test Plan:
cd web/cashtab && npm start
navigate to token screen
click create etoken
enter 'xbt' as the token ticker
enter 'xbt' as the token name
observe that in both cases, the appropriate error appears

Reviewers: bytesofman, emack, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D12735