Allow ApplicationTransactionEditor to figure out whether TYPE_COMMENT is supported or not

Summary: See D17812, etc. We can figure this out by looking at the object carefully. We don't need to go delete all the old TYPE_COMMENT (it doesn't hurt anything) but can nuke it when we see it.

Test Plan:
  - Made a comment in Slowvote (supports commenting).
  - Viewed an Almanac device (does not support commenting).

Reviewers: chad

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D17822

src/applications/slowvote/editor/PhabricatorSlowvoteEditor.php

@@ -14,7 +14,6 @@ final class PhabricatorSlowvoteEditor
   public function getTransactionTypes() {
     $types = parent::getTransactionTypes();
 
-    $types[] = PhabricatorTransactions::TYPE_COMMENT;
     $types[] = PhabricatorTransactions::TYPE_VIEW_POLICY;
 
     $types[] = PhabricatorSlowvoteTransaction::TYPE_QUESTION;

src/applications/transactions/editor/PhabricatorApplicationTransactionEditor.php

@@ -298,6 +298,18 @@ abstract class PhabricatorApplicationTransactionEditor
       }
     }
 
+    if ($template) {
+      try {
+        $comment = $template->getApplicationTransactionCommentObject();
+      } catch (PhutilMethodNotImplementedException $ex) {
+        $comment = null;
+      }
+
+      if ($comment) {
+        $types[] = PhabricatorTransactions::TYPE_COMMENT;
+      }
+    }
+
     return $types;
   }