Add a "View Task" button to HTML mail from Maniphest

Summary:
See downstream <https://phabricator.wikimedia.org/T1050>. Some time ago, we added a "View Revision" button to Differential mail. This hasn't created any problems and generally seems good / desirable.

It isn't trivial to just add everywhere since we need a translation string in each case, but at least add it to Maniphest for now. Going forward, we can fill in more applications as they come up.

Test Plan:
Used `bin/mail show-outbound --id <x> --dump-html`:

{F6470461}

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20561

src/applications/audit/editor/PhabricatorAuditEditor.php

@@ -485,6 +485,10 @@ final class PhabricatorAuditEditor
     return $phids;
   }
 
+  protected function getObjectLinkButtonLabelForMail() {
+    return pht('View Commit');
+  }
+
   protected function buildMailBody(
     PhabricatorLiskDAO $object,
     array $xactions) {

src/applications/differential/editor/DifferentialTransactionEditor.php

@@ -601,6 +601,10 @@ final class DifferentialTransactionEditor
     return $xactions;
   }
 
+  protected function getObjectLinkButtonLabelForMail() {
+    return pht('View Revision');
+  }
+
   protected function buildMailBody(
     PhabricatorLiskDAO $object,
     array $xactions) {
@@ -617,7 +621,7 @@ final class DifferentialTransactionEditor
     $this->addHeadersAndCommentsToMailBody(
       $body,
       $xactions,
-      pht('View Revision'),
+      $this->getObjectLinkButtonLabelForMail($object),
       $revision_uri);
 
     $type_inline = DifferentialTransaction::TYPE_INLINE;

src/applications/maniphest/editor/ManiphestTransactionEditor.php

@@ -206,6 +206,10 @@ final class ManiphestTransactionEditor
       ->setSubject("T{$id}: {$title}");
   }
 
+  protected function getObjectLinkButtonLabelForMail() {
+    return pht('View Task');
+  }
+
   protected function buildMailBody(
     PhabricatorLiskDAO $object,
     array $xactions) {

src/applications/transactions/editor/PhabricatorApplicationTransactionEditor.php

@@ -3417,12 +3417,17 @@ abstract class PhabricatorApplicationTransactionEditor
       ->setViewer($this->requireActor())
       ->setContextObject($object);
 
-    $this->addHeadersAndCommentsToMailBody($body, $xactions);
+    $button_label = $this->getObjectLinkButtonLabelForMail($object);
+
+    $this->addHeadersAndCommentsToMailBody($body, $xactions, $button_label);
     $this->addCustomFieldsToMailBody($body, $object, $xactions);
 
     return $body;
   }
 
+  protected function getObjectLinkButtonLabelForMail() {
+    return null;
+  }
 
   /**
    * @task mail