Local Authentication

[mattrubin]

Based off of https://github.com/mattrubin/Authenticator/pull/219, this PR adds a "screen lock" feature which hides the tokens by default when the app launches and whenever it resigns active. The app then prompts for Face ID, Touch ID, or passcode authentication when the app becomes active.

This PR is ready to be merged for beta testing, but there are several known places this feature can be improved before release:

• A toggle to enable/disable this feature on the settings screen.
• Improved UI and explanation on the lock screen.
• Possibly, the use of a separate window for the lock screen, rather than a modal view controller.
• Thorough testing on devices with and without various combinations of Face ID, Touch ID, or passcode.
• Possibly, the use of security features of the Keychain API to make the stored tokens only accessible to the app after authentication has succeeded.
• Improved UX design for the case where the user disables Face ID, Touch ID, or passcode while the app is running (or is not running but has screen lock enabled in its settings), leaving the app with no system-provided authentication mechanism.

[savyyy001]

Why does it say “please Make Sure systems Device time is correct” when i put in 6 digit code?