captcha: allow for interception

[hjanuschka]

to allow captcha logic to be run BEFORE the submit happens,
in AJAX forms, check if PowerMailFormOverrideAjax exists
in the window. object, if so - call it and wait for it (promise)

this enables a clean interception of the ajax submit
as messing with multiple onSubmit handlers is a nightmare

[hjanuschka]

as a workaround - cloning and replacing the form node seem'd to work

var oldForm = document.querySelector('form');
var newForm = oldForm.cloneNode(true);
oldForm.parentNode.replaceChild(newForm, oldForm);
form = newForm;

edit: but this disables ajax :/

[hjanuschka]

@mschwemer any chances you can take a look?
happy to discuss and change whatever is needed, as long as ajax allow's somehow to intercept/chain into.