Use InputRequired instead of deprecated Required wtform validator

`InputRequired` allows value of `0` as an `IntegerField` value.

As you can see [here](https://bitbucket.org/simplecodes/wtforms/src/acf20942487e18a1b7cbc542e9130bf0adc111b7/wtforms/validators.py?at=default#cl-196), `Required` is deprecated and points to its old implementation now called `DataRequired`. You should use `InputRequired`, though, as it also allows `0` as a valid value for `IntegerField`.