Feature account creation

[webdavis]

Overview of this PR

Development Configuration

• Adds configuration for Development, Staging, and Production environments, all of which are managed using xcconfig files. To switch to a different environment just change the scheme.
• Adds an Enviroment.swift file which exposes environment variables grabbed from the xcconfig files.
• Adds code that detects if you are in the Development environment, and sets up Firebase Emulators accordingly.
• Adds version tracking for the firebase-tools command-line client.

Firebase Account Creation

• Adds an AuthController that uses completion handles to expose Firebase authentication.
• Adds a UserController that can be passed around to ViewControllers to do work on a user object.
• Uses Dependency Injection instead of Singletons for the UserController so that we can write better tests going forward and have multiple users in the future, since this is a collaborative Vacation planning app. For more information on how to pass around data in an application see this article: Passing Data Between View Controllers in iOS: The Definitive Guide.

Removes the need for Hard Coding information

• Adds service and service managers for retrieving configuration from CloudKit.
• Adds protocols that manage these services and managers so that alternatives can be easily changed out in the future, and so that multiple CloudKit Records may be used to store config information. (Basically, we store our API Keys and API URLs in a public cloud database, and this makes it easy to break up the things we store as we add more things going forward.)
• Adds a ConfigController singleton. Since we are only ever reading data it's fine to use a singleton here.
• The pattern used to expose this singleton makes it easy to stub out for testing. For more information on this see this article: Protocol Extensions And Shared Dependency Injection in Swift, an Alternative to Singletons.
• Adds CloudKit subscriptions to automatically fetch Config updates.
• Uses Notifications for CloudKit record updates.

Accounts Controller

-

Things that still need to be setup

• Automate development environment setup. (Didn't have time to get to this!)
• CloudKit error handling still needs some work.
• Firebase error handling still needs to be implemented.
• Firebase Auth API rules need to be setup.
• [weak self] needs to be handled in addNotificationObserver. (Forgot how to do this, just need to Google for a minute most likely.)
• UserDefaults needs to be swapped out for something more secure and reliable.
• Unit/Integration tests still need to be written.
• Probably a bunch of other things that I can't remember at the moment.

[webdavis]

@Erikjimenez314 @hb8man @Kokonut09 -- this is ready for a code review if you would like to take a look at it. See the overview of this above. I did waaaay too much work on this PR. This absolutely should have been broken up into like 4 different PRs.