Preserve "next" URI by using OAuth 'state' parameter

Summary:
When a user clicks a link like /T32 and has to login, redirect them
to the resource once they've authenticated if possible. OAuth has a param
specifically for this, called 'state', so use it if possible. Facebook
supports it but Github does not.

Test Plan:
logged in with facebook after viewing /D20

Reviewed By: aran
Reviewers: aran
CC: aran, epriestley
Differential Revision: 61

src/applications/auth/controller/oauthregistration/base/PhabricatorOAuthRegistrationController.php

@@ -21,6 +21,7 @@ abstract class PhabricatorOAuthRegistrationController
 
   private $oauthProvider;
   private $oauthInfo;
+  private $oauthState;
 
   final public function setOAuthInfo($info) {
     $this->oauthInfo = $info;
@@ -40,4 +41,13 @@ abstract class PhabricatorOAuthRegistrationController
     return $this->oauthProvider;
   }
 
+  final public function setOAuthState($state) {
+    $this->oauthState = $state;
+    return $this;
+  }
+
+  final public function getOAuthState() {
+    return $this->oauthState;
+  }
+
 }