R1:e1657877251b

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…