Allow custom queries to be deleted

Summary:
Ref T2625. Currently, custom saved queries can be edited but not deleted. Allow them to be deleted. Also:

  - Clean up an unused property in `PhabricatorPasteViewController`.
  - Fix an issue with left nav highlighting of builtin queries.
  - Improve submit behavior for edits.
  - Add a cancel button on edits.

Test Plan: Saved, edited and deleted queries.

Reviewers: btrahan, blc

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2625

Differential Revision: https://secure.phabricator.com/D6059

src/applications/paste/controller/PhabricatorPasteViewController.php

@@ -2,13 +2,12 @@
 
 final class PhabricatorPasteViewController extends PhabricatorPasteController {
 
+  private $id;
+
   public function shouldAllowPublic() {
     return true;
   }
 
-  private $id;
-  private $handles;
-
   public function willProcessRequest(array $data) {
     $this->id = $data['id'];
   }