Complete the `sign-in` workflow

[ManasMalla]

• Need to add the username TextField and gender SegmentedButton to complete the ModalBottomSheet to create a new profile.
• Handle the uniqueness of the username in the textfield
• Need to navigate the user to the home screen to finish the sign-in workflow.
• Manage user roles - Attendee, Volunteer, Speaker, Organizer
• Create the profile for new users
• Show the Success state in the bottom sheet when creating profile was successful
• Code the user's profile in the profile screen
• Allow the user to edit the user profile
• Fetch the user's data from Firestore for the profile
• Edit the user's data to Firestore