Implement "Revert to Version" functionality in Phragment

Summary:
This functionality allows users to revert a fragment to a previous version from the history page.

Reverting a version actually creates a new version pointing at the same file as the version being "reverted" to. In this sense it acts pretty much like Git and other distributed VCS where once you have published a commit the only way to undo your changes is to create a new commit that reverts those changes.

Test Plan: Reverted a fragment to a version before it was deleted, then reverted it to when it was deleted and saw the new versions have the correct file PHIDs (including null for the deletion).

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Maniphest Tasks: T4205

Differential Revision: https://secure.phabricator.com/D7738