Prevent "Call webhooks" Herald action from appearing in UI for adapters which can't fire it

Summary:
See <https://discourse.phabricator-community.org/t/herald-webhook-is-not-called-but-herald-transcript-tells-me-the-opposite/1098>.

The "Commit Hook" events don't operate on objects and don't use TransactionEditors. They can't call webhooks in a normal way and currently don't call webhooks at all. Stop offering these actions in the UI.

The  "Outbound Mail" event also fires oddly and likely doesn't make much sense to hook anyway.

Test Plan: Verified that these events no longer offer "Call webhooks", while normal events still do.

Differential Revision: https://secure.phabricator.com/D19061

src/applications/diffusion/herald/HeraldPreCommitAdapter.php

@@ -87,4 +87,8 @@ abstract class HeraldPreCommitAdapter extends HeraldAdapter {
       $this->hookEngine->getRepository()->getProjectPHIDs());
   }
 
+  public function supportsWebhooks() {
+    return false;
+  }
+
 }

src/applications/herald/action/HeraldCallWebhookAction.php

@@ -14,6 +14,10 @@ final class HeraldCallWebhookAction extends HeraldAction {
   }
 
   public function supportsObject($object) {
+    if (!$this->getAdapter()->supportsWebhooks()) {
+      return false;
+    }
+
     return true;
   }
 

src/applications/herald/adapter/HeraldAdapter.php

@@ -1211,6 +1211,11 @@ abstract class HeraldAdapter extends Phobject {
 /* -(  Webhooks  )----------------------------------------------------------- */
 
 
+  public function supportsWebhooks() {
+    return true;
+  }
+
+
   final public function queueWebhook($webhook_phid, $rule_phid) {
     $this->webhookMap[$webhook_phid][] = $rule_phid;
     return $this;

src/applications/metamta/herald/PhabricatorMailOutboundMailHeraldAdapter.php

@@ -64,4 +64,8 @@ final class PhabricatorMailOutboundMailHeraldAdapter
     return pht('Mail %d', $this->getObject()->getID());
   }
 
+  public function supportsWebhooks() {
+    return false;
+  }
+
 }