Avoid PhabricatorApplicationTransactionStructureException on editors not supporting Mute Notifications

Summary:
Do not expose the "Mute Notifications" sidebar menu entry when the underlying Editor for that object type does not support muting notifications (means: creating a transaction of type "core:edge") at all.
This avoids a disappointing `PhabricatorApplicationTransactionStructureException` after two clicks.

Disabling the menu entry while still exposing it makes no sense here as the user could never get it enabled anyway.

```
EXCEPTION: (PhabricatorApplicationTransactionStructureException) Attempting to apply a transaction (of class "PhabricatorFileTransaction", with type "core:edge") which has not been constructed correctly: Transaction has type "core:edge", but that transaction type is not supported by this editor (PhabricatorFileEditor). at [<phorge>/src/applications/transactions/editor/PhabricatorApplicationTransactionEditor.php:1830]
```

```
EXCEPTION: (PhabricatorApplicationTransactionStructureException) Attempting to apply a transaction (of class "LegalpadTransaction", with type "core:edge") which has not been constructed correctly: Transaction has type "core:edge", but that transaction type is not supported by this editor (LegalpadDocumentEditor). at [<phorge>/src/applications/transactions/editor/PhabricatorApplicationTransactionEditor.php:1830]
```

Closes T15378

Test Plan:
* Open various different object types:
** Files: http://phorge.localhost/F1 (not implemented)
** Legalpad: http://phorge.localhost/legalpad/view/1/ (not implemented)
** Maniphest: http://phorge.localhost/T1 (implemented)
* Click "Subscribe" on these various types of objects.
* Click "Mute Notifications" on these various types of objects, remember which objects throw an exception.
* Apply this patch.
* Check that "Mute Notifications" is not exposed for those types of objects that threw an exception.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15378

Differential Revision: https://we.phorge.it/D25730

src/applications/subscriptions/events/PhabricatorSubscriptionsUIEventListener.php

@@ -94,26 +94,33 @@ final class PhabricatorSubscriptionsUIEventListener
       }
     }
 
-    $mute_action = id(new PhabricatorActionView())
-      ->setWorkflow(true)
-      ->setHref('/subscriptions/mute/'.$object->getPHID().'/')
-      ->setDisabled(!$user_phid);
-
-    if (!$is_muted) {
-      $mute_action
-        ->setName(pht('Mute Notifications'))
-        ->setIcon('fa-volume-up');
-    } else {
-      $mute_action
-        ->setName(pht('Unmute Notifications'))
-        ->setIcon('fa-volume-off')
-        ->setColor(PhabricatorActionView::RED);
-    }
-
-
     $actions = $event->getValue('actions');
     $actions[] = $sub_action;
-    $actions[] = $mute_action;
+
+    // Hide "Mute Notifications" in sidebar if not supported by Editor - T15378
+    $supported_editor_transaction_types =
+      array_fill_keys($object->getApplicationTransactionEditor()
+      ->getTransactionTypesForObject($object), true);
+    if (array_key_exists(PhabricatorTransactions::TYPE_EDGE,
+        $supported_editor_transaction_types)) {
+      $mute_action = id(new PhabricatorActionView())
+        ->setWorkflow(true)
+        ->setHref('/subscriptions/mute/'.$object->getPHID().'/')
+        ->setDisabled(!$user_phid);
+
+      if (!$is_muted) {
+        $mute_action
+          ->setName(pht('Mute Notifications'))
+          ->setIcon('fa-volume-up');
+      } else {
+        $mute_action
+          ->setName(pht('Unmute Notifications'))
+          ->setIcon('fa-volume-off')
+          ->setColor(PhabricatorActionView::RED);
+      }
+      $actions[] = $mute_action;
+    }
+
     $event->setValue('actions', $actions);
   }