More intelligent BuildItem allocation

- Set initial value for quantity based on how many parts are left to allocate
- Auto select the StockItem to take from (if there is only one)

InvenTree/templates/modal_form.html

@@ -14,6 +14,9 @@
   
   
   {% crispy form %}
+
+  {% block form_data %}
+  {% endblock %}
 </form>
 
 {% block post_form_content %}