Add "optional" field to BomItem

- Defaults to False
- Indicates that the BomItem is "optional" for a build
- Will be used in the future when calculating if a Build output is fully allocated!

InvenTree/templates/js/bom.html

@@ -169,6 +169,10 @@ function loadBomTable(table, options) {
             // Let's make it a bit more pretty
             text = parseFloat(text);
 
+            if (row.optional) {
+                text += " ({% trans "Optional" %})";    
+            }
+
             if (row.overage) {
                 text += "<small> (+" + row.overage + ")    </small>";
             }