Skip theme defined sizes

When is added size by theme or another plugin (like Woocommerce) should not be override by Simple Image Size. It is little bit confused if image size is defined on two places with different dimensions. There is not correct expection which size is right.

My modification simply disable form fields. After submit form that values are not saved.
But in Media screen are visible, so administrator can see which all image sizes.

This commit doesn't handle migrations. Maybe after update should be disabled by checking "custom" attribute in saved options.