Add the option to keep deleted widgets when serializing items.

Summary:
Via `.serialize({keepDeletedWidgets: true})`. Also fixed some
hint editor cruft.

Test Plan:
subperseus'd, was able to cut and paste widgets within both
the question editor and the hint editors.

Reviewers: aria

Reviewed By: aria

Differential Revision: http://phabricator.khanacademy.org/D14267