Improve JS (remove jQuery)

[pwkip]

Long term plan: remove all jQuery code.

Short term: improve compatibility with other jQuery versions: https://wordpress.org/support/topic/error-with-2-5-6/#post-18237766

[saidul]

const buttonEntries = [ ... jQuery('button', form.$form) ].map(entry => [entry.name, entry.value]);

jQuery does not return iterable data. temporary solution could be using Array.from:

const buttonEntries = [ ... Array.from(jQuery('button', form.$form)) ].map(entry => [entry.name, entry.value]);