R1:0ef599e90676

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…