replace all Job/JT relations with a single M2M credentials relation

Includes backwards compatibility for now-deprecated .credential,
.vault_credential, and .extra_credentials

This is a building block for multi-vault implementation and Alan's saved
launch configurations (both coming soon)

see: https://github.com/ansible/awx/issues/352
see: https://github.com/ansible/awx/issues/169

awx/main/models/unified_jobs.py

@@ -149,6 +149,10 @@ class UnifiedJobTemplate(PolymorphicModel, CommonModelNameNotUnique, Notificatio
         default='ok',
         editable=False,
     )
+    credentials = models.ManyToManyField(
+        'Credential',
+        related_name='%(class)ss',
+    )
     labels = models.ManyToManyField(
         "Label",
         blank=True,
@@ -579,6 +583,10 @@ class UnifiedJob(PolymorphicModel, PasswordFieldsModel, CommonModelNameNotUnique
         on_delete=models.SET_NULL,
         help_text=_('The Rampart/Instance group the job was run under'),
     )
+    credentials = models.ManyToManyField(
+        'Credential',
+        related_name='%(class)ss',
+    )
 
     def get_absolute_url(self, request=None):
         real_instance = self.get_real_instance()