[Cashtab] URGENT patch balance display bug in Safari

Summary: D9283 breaks Cashtab in Safari as Safari does not support lookbehind in regular expressions. This diff implements a more universal approach.

Test Plan: npm test to confirm tests still pass. Load it in Safari if you can but might as well green this if it works.

Reviewers: #bitcoin_abc, Mengerian

Reviewed By: #bitcoin_abc, Mengerian

Differential Revision: https://reviews.bitcoinabc.org/D9286