Allow "Sign with MFA" to be applied as a comment action without requiring "CAN_EDIT"

Summary:
Fixes T13307. We currently require "CAN_EDIT" to sign actions, but it's fine to sign a comment with only "CAN_INTERACT".

Since the actions like "Accept Revision" already work like this, the fix is one line.

Test Plan: {F6488135}

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13307

Differential Revision: https://secure.phabricator.com/D20574

src/applications/auth/engineextension/PhabricatorAuthMFAEditEngineExtension.php

@@ -38,6 +38,7 @@ final class PhabricatorAuthMFAEditEngineExtension
       ->setLabel(pht('MFA'))
       ->setIsFormField(false)
       ->setCommentActionLabel(pht('Sign With MFA'))
+      ->setCanApplyWithoutEditCapability(true)
       ->setCommentActionOrder(12000)
       ->setActionDescription(
         pht('You will be prompted to provide MFA when you submit.'))