Give Buildables a status, populate it, and return it over Conduit

Summary:
Ref T4809. Currently, buildables have a status field but nothing populates it. Populate it:

• When builds change state, update the Buildable state.
• Use the new Buildable state on the web UI.
• Return the new Buildable state from Conduit.

To make it easier to debug/test this:

• Provide bin/harbormaster update Bxxx ... to force foreground update of a Buildable.

Test Plan:

• Used bin/harbormaster update Bxxx --force --trace to update buildables.
• Looked at buidlable list, saw statuses reported properly.
• Used Conduit to read statuses.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4809

Differential Revision: https://secure.phabricator.com/D8799