Remove file(adminLTE.css).
Fix(register.jsp): form style is correct.
Fix(signin.jsp): form style is correct.
Add(RegistrationDataValidator): class methods for validating the username and password.
Refactor(RegistrationValidator): rename to StringValidator, contain only one method - validate.
Refactor(RegistrationDataValidator): rename to RegexStringValidator.
Modify(RegisterServlet): add username and password validation.
Modify(register.jsp): change the registration form to a stable one, add validation error messages under each field.