Don't try to generate a CSRF token for the omnipotent user

Summary:
We can end up here with a stack trace like this, while rendering an embedded Slowvote trying to publish a Feed story:

```
Daemon 43450 STDE [Mon, 08 Jun 2015 22:49:57 +0000] [2015-06-08 22:49:57] EXCEPTION: (PhutilProxyException) Error while executing Task ID 830591. {>} (PhabricatorDataNotAttachedException) Attempting to access attached data on PhabricatorUser (via getAlternateCSRFString()), but the data is not actually attached. Before accessing attachable data on an object, you must load and attach it.
Daemon 43450 STDE [Mon, 08 Jun 2015 22:49:57 +0000]
Daemon 43450 STDE [Mon, 08 Jun 2015 22:49:57 +0000] Data is normally attached by calling the corresponding needX() method on the Query class when the object is loaded. You can also call the corresponding attachX() method explicitly. at [<phabricator>/src/infrastructure/storage/lisk/PhabricatorLiskDAO.php:166]
Daemon 43450 STDE [Mon, 08 Jun 2015 22:49:57 +0000] arcanist(head=master, ref.master=7d15b85a1bc0), phabricator(head=master, ref.master=929f5f22acef), phutil(head=master, ref.master=92882eb9404d)
Daemon 43450 STDE [Mon, 08 Jun 2015 22:49:57 +0000]   #0 <#2> PhabricatorLiskDAO::assertAttached(string) called at [<phabricator>/src/applications/people/storage/PhabricatorUser.php:556]
Daemon 43450 STDE [Mon, 08 Jun 2015 22:49:57 +0000]   #1 <#2> PhabricatorUser::getAlternateCSRFString() called at [<phabricator>/src/applications/people/storage/PhabricatorUser.php:432]
Daemon 43450 STDE [Mon, 08 Jun 2015 22:49:57 +0000]   #2 <#2> PhabricatorUser::generateToken(integer, integer, string, integer) called at [<phabricator>/src/applications/people/storage/PhabricatorUser.php:344]
Daemon 43450 STDE [Mon, 08 Jun 2015 22:49:57 +0000]   #3 <#2> PhabricatorUser::getRawCSRFToken() called at [<phabricator>/src/applications/people/storage/PhabricatorUser.php:357]
Daemon 43450 STDE [Mon, 08 Jun 2015 22:49:57 +0000]   #4 <#2> PhabricatorUser::getCSRFToken() called at [<phabricator>/src/infrastructure/javelin/markup.php:91]
Daemon 43450 STDE [Mon, 08 Jun 2015 22:49:57 +0000]   #5 <#2> phabricator_form(PhabricatorUser, array, array) called at [<phabricator>/src/applications/slowvote/view/SlowvoteEmbedView.php:169]
Daemon 43450 STDE [Mon, 08 Jun 2015 22:49:57 +0000]   #6 <#2> SlowvoteEmbedView::render() called at [<phabricator>/src/view/AphrontView.php:175]
Daemon 43450 STDE [Mon, 08 Jun 2015 22:49:57 +0000]   #7 <#2> AphrontView::producePhutilSafeHTML() called at [<phutil>/src/markup/render.php:133]
Daemon 43450 STDE [Mon, 08 Jun 2015 22:49:57 +0000]   #8 <#2> phutil_escape_html(SlowvoteEmbedView)
Daemon 43450 STDE [Mon, 08 Jun 2015 22:49:57 +0000]   #9 <#2> array_map(string, array) called at [<phutil>/src/markup/engine/remarkup/PhutilRemarkupBlockStorage.php:56]
Daemon 43450 STDE [Mon, 08 Jun 2015 22:49:57 +0000]   #10 <#2> PhutilRemarkupBlockStorage::restore(PhutilSafeHTML, integer) called at [<phutil>/src/markup/engine/PhutilRemarkupEngine.php:299]
Daemon 43450 STDE [Mon, 08 Jun 2015 22:49:57 +0000]   #11 <#2> PhutilRemarkupEngine::restoreText(PhutilSafeHTML, integer) called at [<phutil>/src/markup/engine/PhutilRemarkupEngine.php:295]
Daemon 43450 STDE [Mon, 08 Jun 2015 22:49:57 +0000]   #12 <#2> PhutilRemarkupEngine::postprocessText(array) called at [<phabricator>/src/infrastructure/markup/PhabricatorMarkupEngine.php:138]
Daemon 43450 STDE [Mon, 08 Jun 2015 22:49:57 +0000]   #13 <#2> PhabricatorMarkupEngine::process() called at [<phabricator>/src/applications/feed/story/PhabricatorFeedStory.php:167]
Daemon 43450 STDE [Mon, 08 Jun 2015 22:49:57 +0000]   #14 <#2> PhabricatorFeedStory::loadAllFromRows(array, PhabricatorUser) called at [<phabricator>/src/applications/feed/query/PhabricatorFeedQuery.php:37]
Daemon 43450 STDE [Mon, 08 Jun 2015 22:49:57 +0000]   #15 <#2> PhabricatorFeedQuery::willFilterPage(array) called at [<phabricator>/src/infrastructure/query/policy/PhabricatorPolicyAwareQuery.php:237]
Daemon 43450 STDE [Mon, 08 Jun 2015 22:49:57 +0000]   #16 <#2> PhabricatorPolicyAwareQuery::execute() called at [<phabricator>/src/infrastructure/query/policy/PhabricatorPolicyAwareQuery.php:168]
Daemon 43450 STDE [Mon, 08 Jun 2015 22:49:57 +0000]   #17 <#2> PhabricatorPolicyAwareQuery::executeOne() called at [<phabricator>/src/applications/feed/worker/FeedPushWorker.php:12]
Daemon 43450 STDE [Mon, 08 Jun 2015 22:49:57 +0000]   #18 <#2> FeedPushWorker::loadFeedStory() called at [<phabricator>/src/applications/feed/worker/FeedPublisherWorker.php:6]
Daemon 43450 STDE [Mon, 08 Jun 2015 22:49:57 +0000]   #19 <#2> FeedPublisherWorker::doWork() called at [<phabricator>/src/infrastructure/daemon/workers/PhabricatorWorker.php:91]
Daemon 43450 STDE [Mon, 08 Jun 2015 22:49:57 +0000]   #20 <#2> PhabricatorWorker::executeTask() called at [<phabricator>/src/infrastructure/daemon/workers/storage/PhabricatorWorkerActiveTask.php:162]
Daemon 43450 STDE [Mon, 08 Jun 2015 22:49:57 +0000]   #21 <#2> PhabricatorWorkerActiveTask::executeTask() called at [<phabricator>/src/infrastructure/daemon/workers/PhabricatorTaskmasterDaemon.php:22]
Daemon 43450 STDE [Mon, 08 Jun 2015 22:49:57 +0000]   #22 PhabricatorTaskmasterDaemon::run() called at [<phutil>/src/daemon/PhutilDaemon.php:185]
Daemon 43450 STDE [Mon, 08 Jun 2015 22:49:57 +0000]   #23 PhutilDaemon::execute() called at [<phutil>/scripts/daemon/exec/exec_daemon.php:125]
```

Just return `null`.

Test Plan: Will check that tasks clear in production.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D13218

src/applications/people/storage/PhabricatorUser.php

@@ -345,6 +345,12 @@ final class PhabricatorUser
   }
 
   public function getCSRFToken() {
+    if ($this->isOmnipotent()) {
+      // We may end up here when called from the daemons. The omnipotent user
+      // has no meaningful CSRF token, so just return `null`.
+      return null;
+    }
+
     if ($this->csrfSalt === null) {
       $this->csrfSalt = Filesystem::readRandomCharacters(
         self::CSRF_SALT_LENGTH);