Give Drydock Leases more modern status treatment

Summary:
Depends on D19073. Ref T13073. Give leases a normal header tag and try to wrangle their status constants a bit.

Also, try to capture the "status class" pattern a bit. Since we target PHP 5.2.3 we can't use static:: so the actual subclass is kind of a mess. Not exactly sure if I want to stick with this or not. We could consider targeting PHP 5.3.0 instead to get static:: / late static binding.

Test Plan: Viewed leases and lease lists, saw better and more conventional status information.

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13073

Differential Revision: https://secure.phabricator.com/D19074