Login edits

[jfeng530]

Big update so need you guys to test thoroughly. If possible, send screenshots of how the app looks on iOS

• Moved navigation out of App.js to its own directory
• Navigation is now split into two different stacks: PreAuthStack, PostAuthStack
• App.js now handles persistence of user; will redirect to appropriate navigation stack depending on presence of logged in user
• Login.js styled for more 'Chinese'-themed aesthetics
• Login and signup logic split into two different screens: Login.js, Signup.js
• Removed redundant function to handle login and signup; logic is now handled in App.js and with appropriate functions in respective screens
• Added client-side validations when signing up (checking email and password matches)
• Added LoadingScreen.js to display ActivityIndicator when appropriate
• Custom fonts added and loaded using @use-expo/font
• TextInput bugs fixed (overflowing text, border size)