Virtual part fix (#3265)

* Add 'virtual': False requirement to sales order items

* Adds "virtual" filter for part table

* Adds extra validation to the SalesOrderLineItem model

* Prevent creation of stock items for virtual parts

- Add validation check to clean() method of StockItem model
- Improve validation message for StockItemSerializer class

* Hide "new stock item" button for virtual parts

* Hide more buttons for 'virtual' parts

InvenTree/company/templates/company/detail.html

@@ -14,7 +14,7 @@
             <h4>{% trans "Supplier Parts" %}</h4>
             {% include "spacer.html" %}
             <div class='btn-group' role='group'>
-                {% if roles.purchase_order.add %}
+                {% if roles.purchase_order.add and not part.virtual %}
                     <button class="btn btn-success" id='supplier-part-create' title='{% trans "Create new supplier part" %}'>
                         <span class='fas fa-plus-circle'></span> {% trans "New Supplier Part" %}
                     </button>
@@ -61,7 +61,7 @@
             <h4>{% trans "Manufacturer Parts" %}</h4>
             {% include "spacer.html" %}
             <div class='btn-group' role='group'>
-                {% if roles.purchase_order.add %}
+                {% if roles.purchase_order.add and not part.virtual %}
                     <button type="button" class="btn btn-success" id='manufacturer-part-create' title='{% trans "Create new manufacturer part" %}'>
                         <span class='fas fa-plus-circle'></span> {% trans "New Manufacturer Part" %}
                     </button>