Disable selection of inactive TestPlan on New Test Run page

Fixes #3152

NOTE: for now it is still possible to create a TR for an inactive TP
using the /runs/from-plan/<id>/?c=7 URL.

There is a larger question if we should allow any actions with a TP/TR
once it has reached a 'finalized' state. See #779 for more context.

tcms/testruns/static/testruns/js/mutable.js

@@ -8,7 +8,7 @@ export function pageTestrunsMutableReadyHandler () {
 
     document.getElementById('id_product').onchange = () => {
         $('#id_product').selectpicker('refresh')
-        updateTestPlanSelectFromProduct()
+        updateTestPlanSelectFromProduct({ is_active: true })
     }
 
     document.getElementById('id_test_plan').onchange = () => {