[Cashtab] Add local function to convert hash160 to ecash: address

Summary:
T2812

Add a local function to convert a hash160 string to an ecash: address

This is needed to get eToken minting addresses for the Airdrop screen

Test Plan:
Make sure I'm not overlooking some much easier way to get from string to Uint8Array
npm test

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D12669