[Cashtab] currency.prefixes & currency.tokenPrefixes converted to strings

Summary: Related to T2100. currency.prefixes and currency.tokenPrefixes have been converted from arrays to strings.

Test Plan:
cd web/cashtab
grep -r 'currency.prefixes' src/ should return nothing
grep -r 'currency.tokenPrefixes' should return nothing.

As neither prefix is used in the app, no other testing should be required as changing the type will have no impact on existing code.

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D11741