Keychain and Biometric Login

[RohanNagar]

Easier login is here!

Main Improvements:

• Save email to keychain for easy autofill
• Implement TouchID/FaceID authentication
• Biometric authentication setting to allow user to turn it on or off
• Only show biometric options when the device supports it
• Dynamically show TouchID/FaceID text based on the type of device (e.g. show FaceID text only on iPhone X)

Small fixes:

• Register a new user with their hashed password instead of plaintext
• Capitalize Platform names in settings

Addresses #53