Privacy-enhanced OAuth 2.0: redirecting to the user's wallet

[llorllale]

Part of #10

Our wallet app will be web-based. Assuming we want a generic scheme to redirect the client's request to the user's choice of wallet:

• navigator.registerProtocolHandler() (note: not supported on Safari), or
• if PWA, register it as a handler on the platform where it's installed (can get a bit messy)

[llorllale]

TODO: investigate underlying mechanism of CHAPI to "redirect" to the user's wallet