R1:ed0ba41cd2d0

Allow a HarbormasterBuildMessage to be sent to any object

Summary:
See T13054. This prepares for Buildables to be sent messages ("attach", "done scheduling builds") to fix races between Harbormaster and Differential.

The `buildTargetPHID` is replaced with a `recipientPHID` in the API. An additional change will fix the storage.

In the future, this table could probably also replace `HarbormasterBuildCommand` now, which is approximately the same bus, but for Builds.

Test Plan: Viewed builds with messages.