Implement Login Screen

[RobertBangiyev]

Reference CodePath guide to Logging in

• [x] Create login screen with text boxes for users to put in their username and password, a button to sign in, and a button to register an account
• [x] Take the user to the register screen if Sign up button is clicked
• [x] Use logInInBackground to log the user in if the sign in button is clicked and the credentials are correct
• [x] Take the user to the main screen