Show a more reasonable status element for pull requests

Summary:
Ref T182. Replace the total mess we had before with a sort-of-reasonable element.

This automatically updates using "javascript".

Test Plan:

Used "Land Revision", saw the land status go from "Waiting" -> "Working" -> "Landed" without having to mash reload over and over again.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T182

Differential Revision: https://secure.phabricator.com/D14314