[Cashtab] Show XECX balance

Summary:
Show XECX balance in balance header, if it is non-zero, with link to token page on ticker

Include XECX balance in total fiat balance

Test Plan:
npm test, screenshot

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Subscribers: emack

Differential Revision: https://reviews.bitcoinabc.org/D17504