Rename "status" to "event" in most URIs / classes / etc

Summary: Ref T4375. This doesn't get everything (I figure I'll clean up the actual UI strings when I touch the UIs) but should get the bulk of the URIs and class names and stuff.

Test Plan: Clicked every calendar-related link I could find/grep, they all still seem to work. URIs now say "event".

Reviewers: btrahan, chad

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4375

Differential Revision: https://secure.phabricator.com/D8147