Remember action in Differential comment draft

Summary:
It happens to me quite often that I leave the window with revision (by closing it or by visiting a link from it).
When I return then the comment draft is there so I clowncopterize it but forget that I wanted to take some other action than Comment.

Test Plan: Selected "Add Reviewers", added some reviewers, closed the window, opened it - the action and reviewers were still there.

Reviewers: btrahan, epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3530

resources/sql/patches/draft-metadata.sql

@@ -0,0 +1,4 @@
+ALTER TABLE `{$NAMESPACE}_draft`.`draft`
+ADD `metadata` longtext NOT NULL DEFAULT '' AFTER `draft`;
+
+UPDATE `{$NAMESPACE}_draft`.`draft` SET `metadata` = '[]';