Allow enableFormTracking to capture dynamic form changes

[emiliauskas-fuzzy]

Currently, the form tracker initializes all forms and their elements once during 1st call to window.snowplow('enableFormTracking'). When calling it again it skips any forms that had been processed already do to setting a bool flag to each form and checking it on next calls -

snowplow-javascript-tracker/src/js/forms.js

Line 171 in 090d313

if (formFilter(form) && !form[trackingMarker]) {

This prevents from refreshing existing forms which have dynamically added elements. Here is an example:

1. Go to this page: https://stackblitz.com/edit/angular-8-dynamic-reactive-forms-example

2. In this page choose 1 ticket from the dropdown

3. Open dev console and call this code - just replace the collector url before:
   `
   ;(function () {
   ;(function(p,l,o,w,i,n,g){if(!p[i]){p.GlobalSnowplowNamespace=p.GlobalSnowplowNamespace||[];
   p.GlobalSnowplowNamespace.push(i);p[i]=function(){(p[i].q=p[i].q||[]).push(arguments)
   };p[i].q=p[i].q||[];n=l.createElement(o);g=l.getElementsByTagName(o)[0];n.async=1;
   n.src=w;g.parentNode.insertBefore(n,g)}}(window,document,"script","//d1fc8wv8zag5ca.cloudfront.net/2.10.2/sp.js","snowplow"));

   window.snowplow('newTracker', 'cf1',
   '<<<<<<----enter valid collector url----->>>>>>',
   //'52.49.112.190:8080', // snowplow-mini
   {
   appId: 'test-form-app',
   platform: 'web',
   cookieDomain: '<<<<<<----replace domain----->>>>>>',
   sessionCookieTimeout: 3600,
   post: true
   });
   }());
   snowplow('enableFormTracking');
   `

4. Try to change any value in 1st ticket dropdown and observe if change_form event is fired (it should be)

5. Then choose 2 tickets from the dropdown

6. In dev console call snowplow('enableFormTracking');

7. Try to change some values in 2nd ticket and observe if change_form event is fired (it shouldn't be fired)

My suggested solution would be just moving the if condition 2nd part !form[trackingMarker] to this place

snowplow-javascript-tracker/src/js/forms.js

Line 183 in 090d313

helpers.addEventListener(form, 'submit', getFormSubmissionListener(context));

This will avoid duplicating form submit event listener and will allow refreshing newly added elements in the form. Duplication of existing element listeners will be avoided as it is tracked in a separate bool flag array.

For reference see discussion here: https://discourse.snowplowanalytics.com/t/refreshformtracking-for-capturing-dynamic-form-changes/3209/4