Create AphrontWriteGuard, a backup mechanism for CSRF validation

Summary:
Provide a catchall mechanism to find unprotected writes.

  - Depends on D758.
  - Similar to WriteOnHTTPGet stuff from Facebook's stack.
  - Since we have a small number of storage mechanisms and highly structured
read/write pathways, we can explicitly answer the question "is this page
performing a write?".
  - Never allow writes without CSRF checks.
  - This will probably break some things. That's fine: they're CSRF
vulnerabilities or weird edge cases that we can fix. But don't push to Facebook
for a few days unless you're prepared to deal with this.
  - **>>> MEGADERP: All Conduit write APIs are currently vulnerable to CSRF!
<<<**

Test Plan:
  - Ran some scripts that perform writes (scripts/search indexers), no issues.
  - Performed normal CSRF submits.
  - Added writes to an un-CSRF'd page, got an exception.
  - Executed conduit methods.
  - Did login/logout (this works because the logged-out user validates the
logged-out csrf "token").
  - Did OAuth login.
  - Did OAuth registration.

Reviewers: pedram, andrewjcg, erling, jungejason, tuomaspelkonen, aran,
codeblock
Commenters: pedram
CC: aran, epriestley, pedram
Differential Revision: 777

src/applications/differential/view/revisioncomment/DifferentialRevisionCommentView.php

@@ -129,7 +129,10 @@ final class DifferentialRevisionCommentView extends AphrontView {
         $content = $this->markupEngine->markupText($content);
         if ($comment->getID()) {
           $comment->setCache($content);
+
+          $unguarded = AphrontWriteGuard::beginScopedUnguardedWrites();
           $comment->save();
+          unset($unguarded);
         }
       }
       $content =