Fix PropTypes errors in the console

[masakudamatsu]

The initial value of the most props is '', to use sessionStorage (which allows string values only).

For number props, this causes a PropTypes error.

The solution is to allow '' as one of the expected types:

PropTypes.oneOfType([
  PropTypes.number, 
  PropTypes.oneOf([''])
]),

(The same for boolean props.)

[masakudamatsu]

We change back the PropTypes of the boolean state variable props (cssButtonDisabled, previewButtonDisabled, nextButtonDisabled) to string. As we use the string-only sessionStorage to keep the state variables, we should avoid using boolean.