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. Sent messages with `harbormaster.sendmessage`. Processed messages with `bin/phd debug task`.
Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam
Differential Revision: https://secure.phabricator.com/D19062