Implement step for publishing files as fragments to Phragment in Harbormaster

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