Show event icons in header instead of property list

Summary:
Ref T11326. Currently, events show the icon as a property, like this:

Icon: Default

This is boring and terrible. Show the icon in the header instead:

Also minor cleanup on active/cancel states.

Test Plan: Viewed an event, saw icon.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11326

Differential Revision: https://secure.phabricator.com/D16297