Implement HTTP API to restore old version of a file in a magic-folder

[exarkun]

An older content version of a file can be restored and made the tip for that file by merely modifying the content of the file in the folder (eg copying an old version into the folder over-top of the existing version). However, the intent of this change is not completely transparent to magic-folder. It may be a normal edit or it may be an attempt to roll back the file to an older version.

To be able to make the metadata most closely reflect the user intent, magic-folder can expose an API for performing this operation. Use of the API makes it explicit what the user intent is.