Create a status tool by giving /calendar/ some teeth

Summary: you can now add, edit, and delete status events. also added a "description" to status events and surface it in the big calendar view on mouse hover. some refactoring changes as well to make validation logic centralized within the storage class.

Test Plan: added, edited, deleted. yay.

Reviewers: epriestley, vrana

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T407

Differential Revision: https://secure.phabricator.com/D3810