New current user entity to track logged-in user

[TiareMar]

OVERVIEW OF CHANGES:

• Refactored the InstructorMainScreen for consistency to match the StudentMainScreen
• VERY IMPORTANT: also for consistency, changed the name of the student main screen to "StudentMain" (to match "InstructorMain"). All screens that used "main" have been changed to "StudentMain". If your screen was not in the main branch yet, you need to make this change manually
• Also, new CurrentUser class with static variable currentUser which holds the User object of the current user to address the null user bug currently in Main. See below for more details:

HOW TO USE CurrentUser:

• By introducing the CurrentUser class, we are decoupling knowing who is logged in and the controller/interactor/presenter/etc. flow
• Now the STATIC variable currentUser in the CurrentUser entity class is going to hold the User object of whoever is logged in
• This (hopefully) means that 1) you don't need to access the logged-in user User object until your interactor class for your use case and 2) you don't need to know who is logged in in Main when your controllers/interactors/presenters are initialize (i.e. you can get rid of that parameter)
• So, instead of dragging a User object through your controller to your interactor as a paramecter, you can access the currently logged-in user directly in your interactor by calling CurrentUser.getCurrentUser() (recall that currentUser is a STATIC variable)

[CC-3636]

looks good--make sure to add logic (i.e. how ppl need to change their interactors) -- thank you!!!

[celinelumqr]

code follows clean architecture as far as I can tell and new classes and methods are well-documented