File structure refactoring, fixed AppUser streaming via Provider

[tbeidlershenk]

Changes

1. Changed folder structure to better categorize files and renamed some files / class names to match each other
2. Fixed the AppUser stream in MultiProvider in main.dart in order to stream the user's data from Firestore based on the auth user
3. Boy scout rule'd some files ~ most notably I made FirestoreService and AuthService to have only static methods (we were instancing these classes when we needed to use their methods, it didn't really make sense)
4. Updated Wrapper to map to LoadingAnimation if the AppUser Provider hasn't received data yet

Notes

I think the way I did change # 4 is kind of bad let me know if you don't get what I did or have a better idea of how to do it. Also you can verify that the AppUser stream is working by looking in ProfilePage, it loads the first name of the user which is one of the fields that is only stored in Firestore, not in Firebase Auth

[jeffzheng13]

A few changes from the errors I observed. Also, we probably don't care right now but we will see an error if user created an account without first name and last name set correctly, so we just need to be wary of that.

[jeffzheng13]

Good to go. Merging..