8.2 Admin: have validated data to ensure reliability

[christine-zheng]

( Parent ticket: #41 )

• i.e. each customer that creates an account should only be able to do so once with a single email address.

Have form validation for user login/sign-up

There is also database-level validation on User Model:
User Model for database:
username: not null, not empty and unique,
password: not null and not empty
email: make sure it is unique
userType: ENUM (customer, admin)