@W-15316962/W-15316987: [iOS] Update native login sample app to include User Registration/Forgot Password flows

[JohnsonEricAtSalesforce]

🤘🏻 *Ready For Review! 🎤

This is a companion to the MSDK implementation in https://github.com/forcedotcom/SalesforceMobileSDK-iOS/pull/3728

The most interesting part of the code is a handful of utility methods in the layout to help keep the code from being repetitive.

Here are a few useful screenshots.