Updated modal form code

- Modal form template now uses multipart-data (to allow file uploads)
- Cleaned modal_form.js code (split into separate functions)

InvenTree/templates/modal_form.html

@@ -1,4 +1,4 @@
-<form method="post" action='' class='js-modal-form'>
+<form method="post" action='' class='js-modal-form' enctype="multipart/form-data">
   {% csrf_token %}
   {% load crispy_forms_tags %}
   {% crispy form %}