Implement Face ID / Touch ID + Refactors + The Great Divorce (cleanup unused code)

[tw0po1nt]

• Refactored the LocalAuthentication dependency to more closely match how the LocalAuthentication framework is typically used (i.e. spin up a new context instance). There were issues with the existing approach which shared the same LAContext object between calls.
• Implemented Face ID / Touch ID: Validates the credentials on launch, return from background, when trying to enable / disable the setting, and prior to sending a txn
• Fixed a bug where one could potentially (and errantly) send a memo to a transparent receiver.

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[tw0po1nt]

Self-review complete

[pacu]

Because I've burnt with milk I'm pointing this cow out.

The LAAuthentication API plainly sucks (thanks apple!). You should mock it and test all the possible branches of the possible code with unit tests

Out of scope for a Code Review:

test live on Simulator and also try the same workflow on an iPhone that doesn't have a pin set up.

[pacu]

utACK.

Resolved conversation with @tw0po1nt

[BostonZcash]

In as much as can be tested in the presence of the ZcashLightClientKit error, no issues apart from the below encountered. In an emulator I'm unable to test Face ID unlock specifically, though in lieu of this, the iPhone passcode unlock works as expected.

Not sure if new behaviour or existing:

• In wallets with 0 balance, entering a ZEC amount on the 'Send Money' screen expectedly presents a 'TOP UP WALLET' button instead. However, nothing happens upon tapping it.
• With no value entered, 'Scan a payment code' on this same screen is also non-functional.

Also ran into opportunities for the seed restore flow previously noted which I'll open an issue for.

[tw0po1nt]

Interesting, ok. I knew the scan a payment code wasn't working, as that's ZIP-321 and still todo. The top up wallet, as I recall, used to work. So I'll look into that prior to merging this.