Summary: This adds a build step in Harbormaster for publishing file artifacts as fragments in Phragment.
Test Plan:
Created a build plan with the following steps:
* Lease Host
* Upload Artifact
* Publish Fragment
Ran the build plan against a buildable and saw the fragment get created in Phragment. Ran the plan again and saw the fragment get updated with a new version. Modified the file that got uploaded and ran the plan again, checked the history of the fragment, and saw the differences represented as a Diff-Match-Patch patch.
Reviewers: epriestley, #blessed_reviewers
Reviewed By: epriestley
CC: Korvin, epriestley, aran
Maniphest Tasks: T4205
Differential Revision: https://secure.phabricator.com/D7742
