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