(WIP) API backend + unittests

[justinmoon]

This change adds some some basic REST-like API routes to:

• Manage settings
• Create wallets
• Add signers to wallets
• Create PSBTs
• Sign PSBTs with HWI
• Broadcast fully signed PSBTs

It also saves all files (settings and wallets) to ~/.junction instead of repo directory.

Some unittests were added to test:

• Bitcoin Core watch-only wallet creation & export
• IO

Not deleting the old flask app yet until this and react frontend get merged and reach feature-parity.

@wbobeirne the best way to review this would be to meet up and negotiate how exactly we'd like frontend and backend to be combined.

[wbobeirne]

All of this work is on develop now 👍