Improve escaping of validation JS

[mboynes]

Replace the invalid uses of esc_js() and esc_attr() in class-fieldmanager-util-validation.php, e.g. this block and this block, with more appropriate escaping procedures. The entire file warrants review for updated standards. We should perhaps consider deprecating it in favor of something more modern, ideally that doesn't depend on jQuery.

Props @natebot for noticing.