Make the "All Day Event" control use a checkbox instead of a dropdown

Summary:
This feels a little cleaner:

• Clean up transaction log a bit.
• Use a checkbox instead of a two-option dropdown.

This is a little messy because the browser doesn't send anything if the user submits a form with an un-clicked checkbox.

We now send a dummy value ("Hey, there's definitely a checkbox in this form!") so the server can figure out what to do.

Test Plan:

• Edited all-dayness of an event.
• Viewed transaction log.

Reviewers: chad

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D16776