Don't publish anything for "Disable Mail" / "Enable Mail" on projects

Summary:
Fixes T10466. Currently, clicking "Disable Mail" or "Enable Mail" on a project toggles an edge, but it gets a default "added an edge" story and transaction record.

These are confusing, useless and not interesting, so just hide them.

Test Plan:
  - Before patch: clicked enable/disable mail, saw "added an edge" / "removed an edge" stories in feed and project history.
  - After patch: clicked enable/disable mail, saw nothing in feed or project history.
  - (Note that this patch is not retroactive for feed, so already-published stories won't unpublish.)

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10466

Differential Revision: https://secure.phabricator.com/D15361

src/applications/project/storage/PhabricatorProjectTransaction.php

@@ -69,6 +69,21 @@ final class PhabricatorProjectTransaction
     return parent::getColor();
   }
 
+  public function shouldHide() {
+    switch ($this->getTransactionType()) {
+      case PhabricatorTransactions::TYPE_EDGE:
+        $edge_type = $this->getMetadataValue('edge:type');
+        switch ($edge_type) {
+          case PhabricatorProjectSilencedEdgeType::EDGECONST:
+            return true;
+          default:
+            break;
+        }
+    }
+
+    return parent::shouldHide();
+  }
+
   public function shouldHideForFeed() {
     switch ($this->getTransactionType()) {
       case self::TYPE_HASWORKBOARD: