InputBase: Add minLength, maxLength, required

[olleolleolle]

This PR adds validation settings to individual form elements.

Using HTML attributes, we rely on the browser's input validity features.

Example:

                 <TextInput labelMsg="forms.person.firstName" name="first_name"
                    initialValue={ person.first_name }
                    constraints={ { required: true, maxLength: 20, minLength: 5 } } />

How to make a Pane validatable:

1. add constraints={ { minLength: 5, maxLength: 30, required: true } } to a Form field
2. add the isValid state to the Form containing the field
3. add the onValidityChange(newValidity) event handler to the Form. This will PASS ON the change. Example:

       onValidityChange(newValidity) {
        console.log("PersonForm changed", newValidity);
   
        if (this.props.onValidityChange) {
            this.props.onValidityChange(newValidity);
        }
    }

4. add the onValidityChange(newValidity) event handler to the Pane. This will set the isValid local state. Example:

   onValidityChange(newValidity) {
    console.debug('AddPersonPane onValidityChange', newValidity);
   
     if (newValidity !== this.state.isValid) {
            this.setState({
                isValid: newValidity
            });
        }
    }

5. show or hide the Pane Footer switching on the isValid state. Example:

    renderPaneFooter(data) {
        if (!this.state.isValid) {
            return null;
        }
        
        return (
            <Button className="EditPersonTagPane-saveButton"
                labelMsg="panes.editPersonTag.saveButton"
                onClick={ this.onSubmit.bind(this) }/>
        );
    }

See #43