Show boolean values as checkbox in Studio.

**Ticket:** [BLD-666](https://edx-wiki.atlassian.net/browse/BLD-666)
Boolean values as a setting are quite common, and right now, we display those as a drop down with the options of True and False. It would make much more sense to display them as a checkbox.

**Sandbox:** http://studio.polesye.m.sandbox.edx.org/

@cahrens, @talbs please review.