Add a "filePHID" to HarbormasterBuildLog and copy logs into Files during finalization

Summary: Depends on D19131. Ref T13088. During log finalization, stream the log into Files to support "Download Log", archive to Files, and API access.

Test Plan: Ran write-log and rebuild-log, saw Files objects generate with log content and appropriate permissions.

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13088

Differential Revision: https://secure.phabricator.com/D19132