Fix #297 -- Add Django admin' inline formset support (#300)

As described in #249 and #297 django-select2 does not currently work
with selects added dynamically in the Django admin.

The solution was implemented in #249 and all credit should go to Jurrian
Tromp. I only added a small correction:

```diff
- jqRow.find('.select2-container').remove()
+ jqRow.find('.django-select2').parent().find('.select2-container').remove()
```

to only remove `.select2-container` when it is a sibling of
`.django-select2`. Otherwise the wrong `.select2-container` might get
deleted.

---------

Co-authored-by: Johannes Maron <[email protected]>

django_select2/static/django_select2/django_select2.js

@@ -68,7 +68,11 @@
   }
 
   $(function () {
-    $('.django-select2').djangoSelect2()
+    $('.django-select2').not('[name*=__prefix__]').djangoSelect2()
+
+    document.addEventListener('formset:added', (event) => {
+        $(event.target).find('.django-select2').djangoSelect2()
+    })
   })
 
   return $.fn.djangoSelect2