Remove ForeignKey relations involving CustomInventoryScript

Add migration to carry out corresponding schema change

Add data migration to delete inventory sources
  which are the custom type

Split migration into two files
  data migration touches same rows as schema migration

awx/main/migrations/_rbac.py

@@ -47,6 +47,18 @@ def delete_all_user_roles(apps, schema_editor):
         role.delete()
 
 
+def delete_all_custom_script_roles(apps, schema_editor):
+    ContentType = apps.get_model('contenttypes', "ContentType")
+    Role = apps.get_model('main', "Role")
+    cis_type = ContentType.objects.get(model='custominventoryscript')
+    role_ct = 0
+    for role in Role.objects.filter(content_type=cis_type).iterator():
+        role.delete()
+        role_ct += 1
+    if role_ct:
+        logger.debug('Deleted roles corresponding to custom inventory sources.')
+
+
 UNIFIED_ORG_LOOKUPS = {
     # Job Templates had an implicit organization via their project
     'jobtemplate': 'project',