Add previews to ApplicationTransaction

Summary:
Implements previews for Macros and Pholio.

(Design is nonfinal -- kind of split the difference between `diff_full_view.png`, laziness, and space concerns. Next couple diffs will add more stuff here.)

Test Plan: {F28055}

Reviewers: btrahan, chad

Reviewed By: btrahan

CC: aran, vrana

Maniphest Tasks: T2104

Differential Revision: https://secure.phabricator.com/D4246

src/aphront/AphrontRequest.php

@@ -18,6 +18,7 @@ final class AphrontRequest {
   const TYPE_CONDUIT = '__conduit__';
   const TYPE_WORKFLOW = '__wflow__';
   const TYPE_CONTINUE = '__continue__';
+  const TYPE_PREVIEW = '__preview__';
 
   private $host;
   private $path;
@@ -339,6 +340,10 @@ final class AphrontRequest {
     return $this->isFormPost() && $this->getStr('__continue__');
   }
 
+  public function isPreviewRequest() {
+    return $this->isFormPost() && $this->getStr('__preview__');
+  }
+
   /**
    * Get application request parameters in a flattened form suitable for
    * inclusion in an HTTP request, excluding parameters with special meanings.